Angela - you're seeing the price in Canadian dollars - that explains why US users are seeing $500 and you are seeing it at $628.
At the very bottom of the site is a currency selector. You can change it to USD and you'd see what they are seeing.
Hope that helps!
Mark
tisdaleart.com
PS - It's somewhat confusing that it would show CAD in the search results but USD on the art page itself. It seems like they should be the same but apparently they only do the currency conversion for originals in the search results. I'm not sure if changing that would be considered a bug fix or a suggestion. It might be worth writing to tech support about it and see what their feedback is on that discrepancy. If you go to "about" at the top and then contact us in the drop down menu. After that choose technical support.