If you are happy right now with the sound quality from your speakers at the primary listening position, then go for the sub. The BIC PL-200 is a good value:
https://www.amazon.com/Bic-Acoustech-PL-200-Subwoofer-Black/dp/B00OFXV5IO/.
However, there is a good advantage in saving up $500 to get a sub that has good low 20hz extension for movie usage. The BIC drops off a lot in output by 30hz. You have to move up to a higher class of sub to get that deep extension. If you are interested in that, watch for sales on the SVS PB-1000 or the RBH I-12 subwoofer over the next week. Might be some discounts, but otherwise, they run $500.