The best system is the one that feels natural or real. No infalting of dropping prices jsut to get some fake sense of "balance"
I don't agree, for a variety of reasons. To start with, you're dropping this market onto an already fundamentally unrealistic situation (it would take months, and require an inordinate amount of base real estate, to field so much as a battalion in UFO:AI), so you've got severely distorted demand there. Every piece of equipment that's available at the start of the game should, realistically, be effectively free compared to aircraft or installations. It'd be a mess.
As a result, you'd then get into hideous game balance issues—do you want to have equipment actually cost something, or do you want the player to ever be able to build another base or buy aircraft? Economic issues would make every single researchable earth weapon worthless (bleeding-edge, just-developed laser rifles sell for 2.5 times the cost of an assault rifle that has been in production for decades!), and the process of bringing the cost down should take years.
I know it sounds nice to say "this should be realistic", but it's a bad, bad idea.
~J