Rather than a one-size fits all situation for download options, perhaps integrate with fotoquote or a system that determines the use of the image and therefore the fair market price.  photoshelter.com uses this, and is was one of the big things I really liked.  Ultimately I prefer zenfolio for a lot of reasons, but I really miss this feature. 

Unfortunately, FotoQuote is not open to licensing their calculator to other companies, it appears they have an exclusive deal with Photoshelter. This significantly limits the usefulness of their solution.

There are other similar options that can at least allow the price to vary based on desired usage.  Another example is here: http://www.photographersindex.com/stockprice.htm

Sedetweiler - I hosted at PhotoShelter also and that is one of the things I also liked very much there.

FotoQuote has a standalone version that is reasonable if you are selling your work already.  It at least would allow you to price your images before you uploaded them and adjust your price sheets accordingly.

Alex - those are industry standard prices.  They are pretty much the same at all the stock companies.  What software did digital railroad use?  Wouldn't there be some software somewhere that does the same thing?