With all the speakers I auditioned before buying, I tried both with and without a sub. With some speakers -- big towers like the Focal 948 and Martin Logan ML 60 Xti, the single large driver REL sub I had didn't do all that much, not even for imaging, etc. If I had had a sub which could really reach down there -- 14 hz or 20 hz, then what @simonmoon said about atmospherics would probably have become evident.
That said, every other speaker I listened to was improved by a single sub. Bass notes appeared, image and soundstage enlarged and got more definite, and (most surprising) the tonal richness of mids and upper frequencies were altered in a good way. I'm now on my way to stereo subs before, one day, going with a swarm.
As for price, I think there are good options out there and some have been listed already. I think you could likely get set up pretty nicely for under $2k for two.
That said, every other speaker I listened to was improved by a single sub. Bass notes appeared, image and soundstage enlarged and got more definite, and (most surprising) the tonal richness of mids and upper frequencies were altered in a good way. I'm now on my way to stereo subs before, one day, going with a swarm.
As for price, I think there are good options out there and some have been listed already. I think you could likely get set up pretty nicely for under $2k for two.