Definitely search advertising like AdWords + Yahoo + Bing. Capture the people looking what you are offering. Therefore, they will probably search and be willing to pay. Think about search terms. Worth every cent if done right! And don't believe the "automated" bid stuff in AdWords. Fine tune your own bids and review regularly. Squeeze as much as you can out of your spend and understand your market. Use LOTS of different messages to experiment.
FB is nothing more than broadcast advertising, like TV. Too unreliable and not worth the money. I'm still dubious about click fraud on FB.
Consider your VPS environment and make sure it's redundant (ideally multi-location). Depending on your project, nothing worse than disappointing your paying customers, so best to get the production environment right.
Hold on to as much CASH as you can so that you can react to opportunities as they arise. Depending on your product, some other careful marketing could really pay off - like booths at appropriate conferences.
Make sure you build up lists of interest (email & sms) so you can repeat market later at much lower cost. Conversion isn't just about immediate sales; sometimes it's acquiring targets to sell to later. Depends on the sales cycle and decision process of your offering.
Hope it helps!