Selling prints is a very emotive thing Roger the main difference between pros and ameteurs be that keen or not is that Pros have overheads and business bills to pay where your average ameteur takes pictures for their enjoyment and any money made form print sales normally goes into buying more kit.
For example part of a pros overhead will include such things as:
depriciation on kit (most pros don't play the buy the latest kit when it comes out game)
cost of studio (where applicable) and/or office space
computers and software investments
various taxes PAYE and the like
general cost of running your business such as advertising, business cards, stationery
professional fees such as accountants and various business insurances (pro kit tends to be costlier and has a much harder life, I also have public liability to the tune of £2 million as well as professional indemnity cover)
I have a horribly complicated spreadsheet that I use to calculate my costs
So as an example
Your average keen ameteur buys a 10 x 8 print for £1.00 and gets a frame off the internet for £20.00 total cost £21.00 he sells it for £30.00 and makes £9.00 which he is happy about as it buys him a beer or goes against the latest Canon or Nikon body as it has 1 extra mega pixel because he has been dupped by the mega pixal myth.
Your average pro will probably get the same print and frame for less as he gets a better deal because he buys in bulk so cost to him is £15.00 he sells the print for £45.00 making £30.00 but then take away his overheads and the pro if lucky is left with not far off the same as the ameteur! However the pro will be doing this all day everyday while the ameteur for the most part will be doing their day job for a large part of the week. Thats where we make our money.
I also personally think that pros attention to detail is normally better as well which is why they do it full time in the first place so you are actually getting a better image.
The above costs are purely plucked from the air and are not based on real ones BTW
Sorry for the meandering reply and hope it explains it a little better, note to self dont reply to post while drinking the days first coffee!