Considering the userbase, it likely adds up. I don't imagine they make a ton of money, but they certainly must make some. It does help that the overall design is simple and it's mostly textual content. Text actually uses very little bandwidth and what images they do use are very small.

I do know we certainly buy a lot of paids and keep them up and a good number buy premium paids as well. That stuff adds up over time and DW seems to very much love the RP community and it's likely because we do indeed bring in good income.