The Citroen Ami is 3804€ over 48 months so 79.25€/month, that is my yardstick for affordable.
My punchline was Australians earn more than Romanians and yet they can build cheaper vélomobiles...
You'd need to have mass-market appeal and production to reach this price. For a bicycle, that's near impossible. The Citroën Ami is an absolutely fascinating vehicle, but it is built down to a price. The ideas to realise this are very clever, but it's still something many people find profoundly unappealing.
If you optimized a velomobile concept for utility and price, you'd end up with something that might barely fall under the definition of a velomobile. You need an aerodynamic fairing and some semblance of weather protection. You'd probably start by optimizing the fairing and making it as easy to produce as possible. The rotovelo concept with rotomoulding is clever, but you'd absolutely need to make this far less expensive. An idea could be to use blow moulding. The fairing won't contribute anything to the stability of the vehicle, but it could be somewhat aerodynamic and very cheap. Being made of relatively thin plastic, the fairing would be quite prone to cracking, especially under UV light, but that's none of our concerns, since we're not interested in making anything fun or sustainable, just something cheap to appeal to the mass market primarily by virtue of price. Chassis construction would need to follow some kind of optimized tube construction or superleggera approach. Having any kind of suspension is out of the question, far too expensive. Maybe you could incorporate some clever blow-moulded geometry for dampening the rear wheel, which would of course fail under repeated dynamic load over time, but would at least be very cheap to manufacture. As far as the drivetrain is concerned, this ultra-cheap velomobile needs to have some electric assist, just because people are not willing to spend any money at all if a bicycle doesn't have it. Electric assist by means of a hub-mounted motor is relatively inexpensive and absolutely necessary, as it reduces stress on the chassis and will thus allow the chassis to be built with far less structural soundness in mind. That being said, the rest of the drivetrain is a great annoyance for velomobile design. You need to have chains, idlers, wheels, a movable bottom bracket and/or seat, etc. A movable bottom bracket that needs the chain to be shortened or elongated is just too complex of a technology for normal people, and for the velomobile to be cheap, you can't afford customization or work from the dealer. This will lead us to the requirement that the seat be movable by normal people; maybe we can also find a way to make the bottom bracket movable. Ideally, you wouldn't even make the seat movable to save even more money, but the Sinclair C5 tried that and this was received very poorly. Given the complexity, rigidity requirements, hassle for the buyer and cost for a proper classic bicycle drivetrain, let's just get rid of it alltogether and attach a user-is-awake-sensor, otherwise known as a pedal generator, to the cranks and make it a series hybrid vehicle. Now you don't need such a rigid frame, you just need to be concerned with the wheels not detaching too catastrophically under unexpected dynamic loads. Brakes; drum brakes are expensive, you can get disc calipers for practically nothing. Thus, the front wheels would use cable-actuated disc brakes, the rear wheel would use a cheap rim brake. This makes no sense and is actually dangerous, but people will absolutely require brakes on every wheel for the velomobile to be deemed safe. As far as safety is concerned, blow moulding allows us to integrate a hole for a flagpole as well, so we'd absolutely need to have one. The flag pole and flag are optional extras, but should be somewhat expensive for what they are, so the dealers will be able to make a good profit on them.
But what does that all give us? It gives us a velomobile that consists of the nastiest and cheapest plastic you can find, reminescent of very cheap tool cases. It doesn't have suspension, but maybe some kind of cleverly designed flex in the chassis that at least provides dampening for the first year or so, until it will eventually break. It's powered by an electric hub motor with a battery that's laughably small for the minimum configuration and must be upgraded to be of any use at all. There's a pedal-generator in lieu of a chain drive train that saves complexity and the requirement for the chassis to be reasonably rigid between the crank and the rear wheel. Everything else would be pretty basic, as basic as you can possibly get, but of course there would be an integrated smartphone mount and an app that could display the current speed and battery capacity and maybe provide navigation (for an additional charge). Of course, it would be quite cheap, but would it be fun? You'd probably be capped at around 30 to 35 km/h anyway for safety, which might conveniently be provided by the hub motor as a cleverly marketed form of always-on "regenerative braking". The plastic chassis would dent easily, but dents could be pushed out fairly easily as well. Polypropylene isn't the nicest plastic, but it's actually quite durable and reasonably cheap. You'd need to live with stress-whitening, though. PP can't be painted, so you could have your velomobile in any colour, as long as it's black (for some level of cheap UV protection). Acceleration would probably be nice up to the point where you run out of battery, where the completely useless and absurdly underpowered pedal-generator would essentially force you to abandon your vehicle and come back with a fully-charged battery to continue your journey.
So, would you buy this for around 4000 Euros? I wouldn't even buy it for 1500 Euros to be honest, since it sounds like absolute garbage to me. That might be a fun novelty, but completely useless for anything at all. Well, you might get great fun from crashing into your friends' velomobiles on purpose, but for anything that approaches everyday use, you'd better be off with an electric bicycle for 1500 Euros, which can get you reasonably okay models by now.
Optimizing for price is important, but too aggressive optimization is futile, as you ruin your market. Velomobiles need to be good, not cheap. Don't get me wrong; expensive velomobiles exclude people without money, which is sad and undesirable. However, cheap velomobiles exclude people who want to use them as something other than an expensive novelty.