I haven't looked into this for a while, but this is as I remember it:
First eBay charge a $200 fee for checking our program and database to check it's appropriate useage (etc etc), then it would be around $500 a year for "membership" of the programme, and then $1.60 for each 1,000 times we access the ebay database, so probably (at current useage, although it might go up if we implemented the access into the program) that would be just another $40 or so per year I think.
So a start up cost of around $740, and thereafter just over $500 per year. (plus we have to persuade Mark to spend the time coding it... which he's usually pretty good about, but he's a busy guy

)
Perhaps we are a victim of our own success - I think if we only wanted to check the database for a few items a day it would be much cheaper or even free

but for the volume we'd use it would be classified as commercial.
Mark and I already spend enough on the site for hosting and bandwidth use, after a fair number of hiccups we splashed out on a dedicated server for the Shack, so we're already all out on this one.