Well, one thing seems to be true about the exhaust systems for the 750 and other vintage bikes. The price hasn't seen a drop, other than market fluctuating because of money exchange rates...
No one has came into the market making an equal quality set of pipes for much lower price to impact the asking price.
So, is it a good idea to grab a set of pipes as early as you can afford them, even if you don't need them right then, to have them later when you do, or to resell them later?
I guess it depends upon how much the prices are creeping higher with time if they are worth the investment...