I collect cels and production drawings -- mostly production drawings ... It's notable that while this isn't a production cel (rather a limited edition cel), they actually are still quite collectable and do increase in value (you can find popular Disney and WB limited editions that sell in the thousands of dollars). I personally don't buy limited editions because I think there's something cool about having a piece of the real production. I buy production drawinings because they were (back in the day anyway) actually done by the real artists vs the inked/painted cels which was just some cog in a machine (though prices don't agree with my personal opinion, that's just how I feel). Now, of course, for most big productions, cels don't even exist because they're all inked on the computer

Anyway, back to your problem ... This piece is framed, so that adds about $100 on usually. To be fair, you'd easily spend that much getting it framed yourself unless you actually have a matte cutter and know how to frame. So back that down and you're at around $425 for the piece. Note that this is also signed, which does increase value. So now you have to do some google and eBay hunting to find comparables ... For instance with a couple searches I found http://www.animationusa.com/general/cn002.html which is an actual production cel unframed at $300. Now it's hard to compare because this cel was surrounded by a couple hundred that look almost exactly like it vs yours is only a run of 150. So at that point it's just weighing which one you prefer and whether the price difference is balanced by "feelgoodness". Now if you're looking at it from an investment standpoint, then you want to look at other Johnny Bravo limited editions and see how they increase in value over time (though you don't have much to compare against for him probably). It's obviously a gamble. If you were buying Disney or traditional WB characters, you have a solid investment, but with "niche" characters and cartoons, it's less clear.

So in conclusion I don't think you're necessarily being ripped off, but you're not getting a "deal" by any means. But then you don't buy direct from the cartoon network for bargain basement prices. It's like buying anything from the name brand outlet. Your best bet is to buy off of eBay as long as you do your research. I think I would rather own the production cel that is cheaper than the limited edition, but that's just my personal opinion.

Random Info:
http://www.animationmeat.com/cericel.html
http://www.awn.com/mag/issue2.8/2.8pages/2.8grossfeldcollect.html

Some Galleries I've Bought From (and been happy with):
http://www.thegremlin.com/
http://www.animationusa.com
http://www.animationartgallery.com/
http://www.vegalleries.com

Another thing I recommend is to call a gallery or two ... While they are salespeople to be sure, most genuinely love animation art and will gladly talk to you about it and explain things to you. When I first started collecting a few years ago, that's what I did.