I have not followed much vintage Mitsouko on ebay, but from what I have seen, vintage Mitsouko (as opposed to current) is indeed quite expensive, as most Guerlains. Parfum goes in the hundreds, no matter what the vintage. EDTs depend, but they can be under $100 if not particularly old. Perhaps they're the way to go.
As usual with ebay, you should look around for a couple of weeks at least to see what the prevailing prices are, and then decide whether you want to go this route. Remember that it is very difficult to understand the vintage from the photo. Also, there are very many stock photos, some may look vintage, but they're actually selling the new stuff, so don't trust them unless they are clearly photos of the actual bottle in the house.
Also, remember that usually Guerlains have many followers who, for reasons unknown, bid at the very last minute, so the price shoots up literally in the last minute or so. As an economist, I don't follow this strategy, I just put in my reservation price and let go. Which means of course that I am always outbid.