I support the idea of introducing floating market prices for airplanes. I've got a few thoughts to add.
Calculating the price changes
Two quick thoughts on how AS could calculate the prices.
First, I think it would be important to have different discount/surcharge rates for different types of aircraft. Boeing and Airbus manufacture dozens of narrowbodies every month in real life. Comparatively, they rarely manufacture more than a dozen widebodies a month - the 787 is only 14p.m. at the moment, and the A350 is at 10p.m. I think there are a range of ways you could model this. The simplest might be to have different rates for prop, regional jet, narrowbody and widebody families. For example, you might get a 1% surcharge for every: 25 props, 50 regional jets, 100 narrowbodies, or 50 widebodies of a particular kind built (I'm assuming that the narrowbody market is the largest, but I could be wrong there).
Second, for the sake of realism, there might be some value in linking the discounts for variants of the same aircraft model, at least in part. For example, if Airbus is selling 1,000 A319 light (enhanced) narrowbodies per week, and 10 A319 LR (enhanced) per week, then realistically I don't think Airbus would be heavily discounting their LR jets - the jets are built on the same production lines, so they would probably just ramp up production of their popular models. Maybe they'd offer a small discount (or apply less of a surcharge) for their LR models, but probably not a huge one. The simplest way to do this would be simply to combine all of the aircraft in the same family (e.g. all A319 models; or all 737 MAX 8s) for counting purposes.
Leasing v buying
Three thoughts on the points AK raises about leasing vs buying.
First, I totally support a more realistic lease market that includes fixed-term leases, variable deposits, and variable lease rates based (in part) on the term of the lease and size of the deposit. I'd also be in favour of including break fees in addition to (or even as an alternative to) deposits. Start-up airlines often use leases as a way to grow quickly without having to invest significant capital. Allowing airlines to sign up to multiple long-term leases with large break fees, but low deposits, might help new players.
Second, however, I'm not sure that it is true that airlines cannot get significant discounts for making large lease orders. Leasing companies themselves often receive discounts when they purchase large numbers of planes. Leasing companies often pass some of those discounts on to major customers that effectively underwrite their investment by agreeing to lease a substantial number of planes from them. In fact, in many cases, major airlines will have negotiated leases with the leasing companies before the leasing company places an order with the manufacturer - when airlines commit to those leases, it gives the leasing companies confidence to place large orders, which attract big discounts. Also, in real life, the sale-and-leaseback arrangement is pretty common - airlines buy a whole stack of planes from the manufacturer (which attracts a nice discount), but then sell the plane to a leasing company and lease it back. AS doesn't simulate sale-and-leasebacks, but a simple way to model them would be to keep some form of discount for bulk leases.
Third, I think it is important to distinguish between the discount that one airline (say Emirates) gets for making a large order, compared with the surcharge/discount Airbus offers to all airlines for their very popular aircraft (e.g. the A321neo) versus their unpopular aircraft (e.g. the A380, these days). I think airlines should still get a discount for making large, individual orders, in addition to the surcharge/discount based on the overall demand for a particular kind of plane.
Production rates
I like matth's idea, to also allow production rates to increase/decrease based on demand. It would make the game more realistic (e.g. 747, 777 and A380 rates are being cut right now as demand falls, while narrowbody and big-twin rates are going through the roof!). At the moment, there are some planes (mostly Russian, from memory) that actually have ok economics in AS, but that are extremely unpopular because of the slow production rates. If those planes became popular, it would be realistic for the manufacturers to expand their production lines, which in turn would make it viable for players to adopt those types of planes.
There are two things we would need to be careful about, when scaling up productions lines. First of all, because AS doesn't have production slots, it might be necessary to have diminishing returns on production rate increases - otherwise big airlines could be getting 10-20 A320s and 10-20 737s delivered every day! Second, and closely related, it might be important to think about how the production rate is linked to the % discount, and also back into future production rates. For example, if A320s can be built every 10 hours, but Tu-214s can only be built every 144 hours, you could end up with a vicious cycle - the price and production rate of A320s is skyrocketing because they can be built so fast, and the price of Tu-214s is plummeting purely because nobody can buy them fast enough to reach (for example) 100 planes/week.
It might be possible to link the discount/surcharge and production rate changes to the current production rate. For example, if Boeing has invested in lots of new factories to double its production rate, then Boeing would need to be selling double the number of jets before it considered increasing the production rate again - in other words, Boeing would only expand capacity further if it had reached capacity. If Boeing only sold 1.5x as many jets, it would either be selling of its factories (slashing production) or offering big discounts.
