There are plenty in the world of art photography that go for much more than that, depending on the photo, the artist, and the run. I've recently seen a limited edition photo selling for over 2,500 pounds (by an artist from England), which is well over $5,000 American.
What makes the price so high in this instance, I think, is not just the quality of the photograph (which is high), who the photographer is, or the fact that it's signed, but that it is a limited edition with a very small run of 50. If you're buyer number 51, you're out of luck.
Although I agree it's a high price to start (considering Zach is not a world famous artist, but known only from television), the price may go down depending on the interest he receives. However, there are some in the art world who may buy it because it's a signed, authenticated, limited edition (of only 50), making it more valuable than say, someone's amateur pic of dolphins they took on vacation.
There are limited edition art photographs that sell in this range, as well as many that are both higher and lower in price (anywhere from thousands of dollars to as little as just under 100)-- depending on the above, the artist, and, most importantly, the 'limit' of the run.
Just to add: considering how much some of the stuff from houseguests is going for on ebay (pretty ridiculous price for a t-shirt, etc., don't you think?), I wouldn't be at all surprised if Zach was quite successful in selling his art.