By the time you've paid for the server, the time to write the wrapper, and the cost of maintaining it, $10 per month is actually pretty good if screengrabs are an inconsequential part of your UI.
Exactly! That's why this is totally worth it. Not only do most people not have the skills but it's also not so important to their core product that they need to spend all that time and effort on rolling their own. You have (or maybe you do have) no idea how many things can go wrong with these sorts of things. Both wkhtmltopdf and webkit2png are two pieces of software that have given me more trouble overall than all the time I've ever spent maintaining production and dev environments for any other purpose. This site is totally worth the $10 bucks like you say.