Subwoofer suggestions for budget system?

I owned the Magnepans .7 and bass supported these with two RELs Quake MKII. Even these 'little' Maggies needed just a hint of extra bass from these subs, They  have a decent set of controls to set them up to taste. This combination really sounded great for me. Of course this also depends on the kind of music you listen to. I'd start with one (second hand) REL and later, after some saving, when your budget allows it again, buy the second one. I'm quite shure you won't be disappointed. I wish you luck in choosing!

$200-300 is pretty low. Subs are heavy to ship. If this was me, I'd search US AudioMart using my zip code and a 100 mile radius to see who nearby wanted to sell a subwoofer; then, assuming it looked ok as a deal, I'd arrange a local pickup. This strategy avoids the costs and risks of shipping and maximizes the value. 

what type of music do you listen to that you are missing the low frequencies for?

I am in the same boat - low budget, considering a sub - but I am not sure it would make a significant difference for me

@hilde45 shipping for a 12" sub with UPS ground anywhere in the lower 48 < $40. Not worth driving around for hours

I think shipping is free? 

https://www.parts-express.com/8-Wireless-Subwoofer-Package-with-Dayton-Audio-SUB-800-300-5791

Any subwoofer is better than no subwoofer.