Bit of background - lots of Amazon sellers use a software called tactical arbitrage that scrapes retailers to get prices and compares prices to Amazon. It comes with a couple hundred sites built in, and the ability to add new sites using custom xpaths. I made a chrome extension that lets you point and click on arbitrary sites to automatically create an xpath file that would be compatible with this software. Charged $199 for it, although I had some launch specials at $149 and above.
Still have a handful of organic sales a year, although it's not really worth the time spent in support anymore. In retrospect I should have made it $99 upfront plus $10/month or something and provided ongoing support.