Useful service, I would use it assuming availability is good (I have jerryrigged something similar myself in the past and it was a real pain in the ass to get working smoothly). Suggested pricing seems way off though (I know it's free right now) - how did you figure out what was reasonable?
It seems like your target customer is someone who's not going to set up PhantomJS on their own, right? For 500 dollars I'll figure out how to set it up.
Another comment: the intro two paragraphs are probably more technical than they should be. The "what for" should be above the "headless browser" section; you might even put all of that stuff on a second "technical details" page to not scare people away. You're not selling "headless browsing" (anyone knows that they want that can probably set it up), you're selling "testing" and "screenshots" (and maybe other services too).
A few tutorials on how to do various tasks with your service would help a lot too (beyond the one-liners).
I'm a developer, which I think everyone in your target market is, so let's assume I could probably roll my own version if I could dedicate a couple of days to it (that would be consistent with my previous experience). I'd expect you to set your pricing low enough that I should never even consider trying to roll my own, so low I never even bother to lookup the docs to see how hard it might be. As pseut says, the upper tiers are so expensive that you just motivated anyone considering that volume to go off and do the build/buy analysis.
I hope this is constructive feedback. There are lots of other good suggestions here on how to make your landing page convert better. If you take all the feedback on board, I do think you have a service that can provide value.