If a cheap frame says it's made of Chromoly, but doesn't specify more, how would it compare with something like Reynolds 520 frame? In terms of weight and ride?

I'm thinking of the $100 Nashbar frames....

Nashbar might be able to tell you something about the frame but there are a lot of variables that go into frame construction. Weight can tell you something about the quality of the steel used for the bike. Would not surprise me if that $100 nashbar frame is on the heavy side for a chrome moly frame.

Even basic CrMo steel can be properly used in a range of gauges in frame tubes reaching all the way down to double butted 0.80-0.50-0.80 mm thicknesses. That is comparable to the old Columbus SL. But Columbus SP was much heavier yet made from the same material. So the steel type is no automatic indicator of tube weight and therefore frame weight.

I would first look at what Chromoly is and isn't.

https://en.wikipedia.org/wiki/41xx_steel

Try this site and you will see that the Reynolds chromoly is the 4130 variety.

I have a Cervelo Prodigy and its Columbus chromoly. So is the Cervelo Renaissance. But I don't know the SAE grade.