How long is a piece of string? 
hard one. everyone will offer different opinions.
Having said that it comes down to how much the customer is willing to pay to have your work on the walls. It doesn't relate to cost of production it's more about how desirable your work is.... think underwater housings.. are they really worth what we pay? But we like them a lot so we bring out the credit card 
Was the customer on a dive trip with you.. if so they have personal connection with you the photographer.. that helps... They'll have something they can talk about when friends come over for dinner... "I met this photographer on my last dive trip to the wherevers ... fabulous work...."
or ...Did they just see your pictures on the web and decided they liked them? The actual answers to those questions don't really matter at all, its more that their motivation for purchase will influence the price point they are willing to respond to.
I'm assuming print only at this stage as framing is another equation all together.
From your question it sounds like you haven't sold too many prints, so I'd say $150-$200 each sounds like a start.
While you might not be David Doubilet, or Chris Newbert... your work is obviously desirable, otherwise they wouldn't have asked.
Can't help with the online print solution in the states sorry.