I would guess their costs are several steps above Wikipedia. Yes, Wikipedia is pretty much ubiquitous, but I know personally that I only visit Wikipedia occasionally (i.e. not every day) and I only visit a few pages. On the other hand, when I was an active redditor, I'd visit hundreds of reddit pages every day.
Tack on the part where bandwidth is more expensive than equivalent amounts of storage, and you've got yourself a reddit that costs a lot more than Wikipedia.
I'm willing to bet Wikipedia uses far more bandwidth than Reddit. Reddit barely hosts any images, it's mostly just all text, plus plenty of sites hotlink Wikipedia images.
This is a good point, reddit has plenty of images but they are generally hosted elsewhere. The images they do host directly are fairly static, and can be cached.
Tack on the part where bandwidth is more expensive than equivalent amounts of storage, and you've got yourself a reddit that costs a lot more than Wikipedia.