sub for Studio-20s?

OK, so I think I want a sub for my Paradigm Studio-20s .... to filkl in with more bass. I see some subs costing more than my Studio 20s!!

What is a good basic sub that will do well in my case?

A used REL.
Paradigm makes some excellent subs. I'd look there.
Sunfire makes some cool subs that work well with your studio 20's. That was one of the first nice systems I had a chance to play with at my local dealer. (Of course, this was 10 years ago when dealers still existed)
I've a paradigm sub one along with the studio 60s and 10s. I really like it annd think it is as good as the REL and others I heard in the price range.
Note that the REL uses a slightly different technology than most other subs with their recommended "high level" input that takes a very high impedence signal from the speaker lugs of the amp itself (a tiny, irrelevant load is seen by the amp). This works by "summing" both channels for the low end and preserves the overall tone of the main speakers and amp by not having to mess with them by filtering the signal in any way. An added plus is you can use one long wire run to the sub with less loss than a pair of single ended cables run from the preamp, allowing more flexibility in placing the sub where it sounds best to you (and in my case away from the TT and rack). I use an older REL Q150E bought for a fraction of its original cost and it works very well...sort of amazing actually.
Try to find an REL T5 to demo. It has some nice features that would help it integrate well with small speakers. If your goal isn't home theater I suspect that it would be a nice choice.
There are used RELs out there for a fraction of new ones. I saw one like mine with a cable recently for about $300.