On the other hand, the price looks high for a site that gets that much traffic. I can pretty consistently create SEO-oriented sites that get that much traffic & revenue in 1-2 yrs with an investment of my time that's more like $10-15k. (Working pretty intensely for a month, and mostly waiting for the link network to mature)
(Of course, I've gotten tired of that and now I'm trying to break into making sites that are 10-100x bigger than that)
Changing a successful site is always risky. There are two risks. (1) is that you might go from something that works to something that doesn't work, and (2) in theory the ideal way to drive up your traffic would be to make small incremental improvements, watch your results, then make more changes. Google's very happy for you to do that if you're using AdWords, but they don't like you doing that in SEO, so there are things built into the system that can (sometimes) zap your ratings if you try to revamp an old site.
Personally I can think of better things to do if I had $100k sitting around, but if you like this site you should think about trying to negotiate the price down. I think a multiple of 8 times earnings is fair, so something like $25k more like it.
Of course, his operating costs are low, so like your average domainer, he can probably sit around a long time to find the guy who'll pay too much.