It’s only once you start developing your web app and have passed the point of “Will it work” do you start to consider icons. Having nice icons used appropriately is one of the things that differentiates professionally-built sites. While it is worth paying for icons, its also nice to start with free ones. When you see what you like and what works for you, then buy the larger set or vector representation. Prices are all over the maps, from $25 to $500 depending on the vendor and icon set.