What they should do is do it the same as Wikipedia. I'm not sure what Reddit's costs are like, but if they did a donation drive where it was a choice of donating or more ads, I bet the community would chip in.
The biggest problem with Reddit Gold is it seems like a feature you can live without (especially if you use reddit enhancement suite). They don't emphasize enough that it's a donation to keep the site going.
Reddit... well it's mainly a time sink.
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.