I have several green bta's that color. I purchased mine from a local store, and they were under power compacts, and looking a little bleached. BUT, the tanks were shallow 20 gallons. Your BTA looks fine, I wouldn't think he was bleaching.
I placed mine in a 75 gallon, with two 250 watt HQI's, on the bottom, and allowed them to move where they would be most comfortable. Since then, I've "fragged" the largest one twice. BUT ONLY after he had been in my tank for a year. All are doing well, but they are great indicators for light quality, and even water quality If your light is bad, over time they will look white, but if your light is adequate, they will remain green.
Another sign of health is the stickiness of tentacles, but be careful when touching them, due to pH sensitivity. If they are sticky, and eating, and his foot looks dark, he should be ok.
Also look for signs of gaping mouths. If the mouth is hung open, and it looks as if it's insides are coming out, begina regiment of garlic extract/selcon soaked foods with cyclopeeze. Resurrecting these animals have become a part of my hobby here for a local store.
Overall, I think BTA's are great animals as far as anemones are concerned. If you research them you'll read that many people have little luck with them not living longer than a year most times. If you keep him happy, and out of powerheads, overflows, and away from predators, you'll be fine. Just keep an eye on pH, alkalinity, and overall water/light quality.
Well, I'm rambling, and hope I was helpful.
As far as price, however, expect anywhere from $20 to $45, depending on store.
izzy