Way back in the day, I had no budget, so I used OSS web themes with very light customization. Then when I actually started selling appreciable amounts of software, I had that website redone by freelancers (twice). It has not been my experience that cutting-edge web design has made huge differences to sales in the sort of markets I tend to operate in, though, so these days I mostly just get a themeforest/etc theme, have designers add in decent graphics (if required), and then do a bit of munging by myself.
Designers, cover your ears for a moment: There are many businesses which have sold many millions of dollars of product, including in our industry, with web design which is less impressive than things you can get on Themeforest/WooThemes/etc for whole tens of dollars. (This is not just applicable to marketing sites, by the way. There are plenty of applications which end up looking like they were designed by a software engineer which, for $15, could have had that same engineer just extract a Rails template from a Themeforest project and end up looking 100x better. Search for [admin] on Themeforest and feast your eyes.)