For a 400PS, the Ebay prices have been slowly climbing up. Not too long ago one went to $600 but the reserve was not met, I have seen them sell for $700. Mine was $500 and change, but that was a few years ago. My tech said the components alone should be worth $1200, he couldn't fathom why a vintage, all tube, hand wired Fender with more power than an SVT should be so cheap. Even at $700, if it is in good working order and does not need tubes, I'd think it was worth it - try to get a '71 SVT for that!
On the flip side, $100 for one that didn't work could be too much. Very risky Ebay buys unless the seller is both trustworthy and knowledgeable about the amp. Rich at Time Electronics has commented he has never had one in for service that had bad transformers - but if you did get one with blown transformers, you might as well throw it out, because the only place you'd get a replacement transformer would be in another 400PS. Even Mercury Magnetics, the big name in boutique transformers, has listed in their online catalog under 400PS, not their replacements for sale, but a want ad seeking originals!
As much as I dig mine, I don't tell everybody to run out and buy one. They are their own animal, unlike other tube amps in some ways. Even I don't use mine exclusively - for most of the songs I play on my 4 string, I like either an overdriven British tone or a murky Mesa Boogie tone, where the Fender's one channel is a bit too smooth and subdued and the other channel is too clear and modern. That clean, modern channel is amazing for what I want from my 5 string though. Plus it's nice to know you COULD run 3 8X10s. I've often wondered what a nice Strat would sound like through it, with 3 (or 6!) full stacks.