I recently set up a smug mug pro account so i could link up images from my wordpress blog to a platform where they could be purchased.

However I've only enabled prints to be ordered, so I haven't set up prices for digital downloads.

It's of little surprise that I've not sold any prints in the first 10 days of my trial, but I'm now wondering if I've cut off my greatest chance of sales by not allowing the files to be purchased digitally?

Is that the most common form of images purchases these days, i.e. electronic file purchase?

If so, what is a good pricing structure for commercial downloads? (I see little point in personal downloads)
$20 for web, $50 for low res (1MP) and $100 for high res (4MP) seems like a fair starting point?
How do you think that compares to typical stock photo sites?