[narwan]

Well, if the IFV is capable of killing the Leo 1 too (with ATGM's and/or HEAT rounds) and can carry infantry internally and is (several sizes) smaller (=harder to spot and hit) shouldn't it actually be more expensive (assuming sights etc are of more or less equal quality)?

Sure, MBT's will sometimes survive an attack that an IFV won't. But most MBT's are nearly as vulnerable to modern AT weapons as IFV's are. If you hit a Leo 1 with an ATGM, lets say one with penetration 100, you have an overkill penetration of about 70. If you hit the IFV with it you'd have an overkill penetration of about 90. Not that much of a difference. Even if the Leo had double the armor it wouldn't make that much difference. Now if you're firing with weapons with far less or no overkill the tanks armor is indeed worth more, relatively speaking. But that's not as much a factor of the target (the LEO) as it is dependent on the firer.
Certain very modern MBT's are very well protected indeed. But those are vulnerable to TA weapons and airlaunched weapons. Does that mean TA weapons and attack helicopter should have their cost increased cause they can take out an expensive MBT (while the MBT can't hurt it back)? No, it doesn't. It means you have to put the right system against the right other one. One consequence is that AIFV's when put against same generation MBT's are more often toast than not. The solution is to carry a couple of TA capable AT teams in them. Or have an attack helicopter in support, etc.

You say that no AIFV should cost as much as an MBT. So a Bradley should be cheaper than a T34/85 or vanilla T55? It's not as simple as you make it out to be.