As a photographer, I'd have to say it depends. I'd have to really like it, because I've got plenty of photos I have taken that I like quite a bit.
http://farm1.static.flickr.com/25/54517169_e433bed516_m.jpg

For a 12x18 Giclee print, I'd probably pay $50-80 if I really liked it. I'm not the kind of person that will pay the frequently asked for $300 for an image, but those people exist.

Figure perhaps $150 for a framed version, maybe a bit more if the frame is very nice. Enough to cover the cost of a frame and mat and add some profit. But I'd probably just do that myself, being a photographer.

For an 8x12, I'd pay perhaps $40 for a bare print, $100-120 for a framed version.

I tend to charge about $25 for a 12x18 print, myself, but I'm not making a living oof this stuff, and the prints I give are on Kodak Endura Pro lustre paper - it's a professional paper, but not quite the same as the high end giclee prints.

If you're looking at selling prints yourself, I'd actually start with somewhat higher prices and see if people bite. You can always lower them later.

If you're selling at an art/craft fair, though, you'll have to keep prices very low to be able to sell stuff. Most of the people at those aren't looking to buy $300 photo prints, but may pay $25 for something that cost you $10 to make. Don't expect real profit in doing shows like that.

it is all relative, to me it depends on how easy or available the subject in the picture is. If the subject is rare or hard to get, like say climbing a mountain to get that one shot that will never be repeated then that picture is worth a lot more than a sunset off the back step. Same with the frame, there are cheap frames and expensive frames. For me I would rather buy the print and get my own frame. Also there is composition, is the picture looking the way you like, as in focus, angle, clarity, lighting. And of course the quality of the final product. The end of it though depends on you, if you like something and it is within reach (price wise) then you will probably buy it. I have seen prices range from 25 to 300 dollars. There is no industry standard for the price of a picute.