How much do you have to spend, to get a good Sub?

Not enough info for an informed decision. I would need to know your requirements, the room size,your loudspeakers, and current sub before I could make any kind of recommendation.

I'm in love with my new JL Audio F113. I actually got it off of this site for about a grand off retail. I paid around $2700.

I have a pair of Eminent 8B speakers with a room size of 13X22 FT. I also have a 12" D-Box Sub right now.

Shelby+Kroll Woofer Monitors are designed for use as a music sub. 10" 300w Bash amp that will play fast, low and clean.

For music, I really enjoy my Vandersteen 2Wq. I have a pair of them now, but started with one. It is designed for music, not home theater. You should review the product info on the Vandersteen web site, since this sub has a unique hook-up scheme. This helps to smoothly integrate your mains with the sub. You will need separate amp and preamp, or an integrated amp with pre-outs and power-amp inputs. The sub costs about $1500 new, and you will need to purchase an in-line filter for another $125 afterwards. It has adjustable Q, but no room EQ. Instead, it is designed for best performance when placed in a corner, which is where most people put their subs anyway. Also, your mains will have to have decent output down to 40Hz. I like these so much that I base all system upgrades around them. They are in my rig to stay.