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?
The price is exorbitant compared to the compute cost associated with each request, so let's assume that the value you provide here is that every other developer doesn't have to invest the setup time to build it themselves and keep it running smoothly.
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.
I'm not really a potential customer, but I'll jump in: $199 and $499 per month shocked me, and the range $29 to $499 completely confused me about your target customer. You should have an option marked "FREE" and in the same text as the paid options, and not just "free beta" but "free trial".
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).
Late addition: saying "I'm not really a potential customer" isn't quite right. I'm putting together a hobby-project website with a lot of SVG images and have been thinking of looking into PhantomJS to generate pngs or gifs for browsers where svg isn't supported (preemptively in case this sounds stupid: that's way down the road, so I don't know if what I'm describing makes sense). I read your post and thought that I would be completely happy to spend a dollar or two a month (expected non-svg traffic is basically zero, but I'd like to support twitter cards) to support your project and generate the images on the fly using your service as a CGI, but 1) the pricing convinced me that the answer is "no" and 2) I'm no closer to knowing if what I want makes sense.
A few tutorials on how to do various tasks with your service would help a lot too (beyond the one-liners).
