I've never been an animation collector, but I've worked in the comics collector arena. You should never buy such things as an investment. They aren't going to increase in value significantly, unless Johnny Bravo becomes some stellar animation star, which is wildly unlikely. In fact, the only reason that original Scooby-Doo cels would be worth much is more because they weren't sold off in large quanitites like cels these days often are.

That being said, you should collect things because you like them. That in itself usually, in fact, presupposes that you aren't likely to resell them anyway.

I'd guess that it's overpriced, but not by a lot. The WB store around here sold cels and they were often about as expensive as that, and they were for original well-established characters. Keep in mind that they say they have 150 of them, so they can't be guaranteeing that you'll get the one that's pictured, unless they're not really selling cels but repro lithographs or something. (I think they are, since they say they're hand-painted. Then again, they claim that the Thomas Kinkade paintings they sell are hand-painted, but they're repros spot-touched by assembly liners. It's also possible that they're cels in the notion that they're painted on acetate, but not used for production, though I think that's also not the case.) It's also worth pointing out that it's probably got a repro background, unless JB is non-traditionally animated, which I doubt.

To sum up, I don't honestly know what a good price would be, but it's definitely not a good financial investment. Once you figure out how much you want to spend, buy it because you like it, not because you think one day it'll be worth more. After all, remember that the reason that such things become valuable is because people like them, and the fact that you like it is a much more sound criterion for that than some obscure intellectualized judgement.