Good bass is expensive. While you can buy a modest set of new monitors today for $500 to $1,000 that don’t absolutely suck and can give you plenty of hi fi enjoyment with decent electronics ahead if them, the same cannot be said for a subwoofer. I have not heard any sub recently that retail for below a grand that I consider adequate. And if your room is large, you can at least double that. If you want to smooth out room effects you can double that again and buy at least two.
The plus side of subs in addition to providing bass extension almost no passive set of speakers, floor standers or monitors, can provide, is that you can use stand mounts and in principle get both the imaging advantages and full frequency bass. The cost is that setting up subs to fill in higher bass frequencies can be tricky.