10-12k is perfect...You could probably get a new sub with a new amp, branded...
Just go to any shop...
I think these audio freaks will recommend you one very soon...Or you look into threads(use search function).
10k is good budget...btw, the speakers you have are probably 2 way speakers...If you can expand budget and sell ur current speakers for some decent amount, you can get new speakers as well, the improvement in sound quality will shock you...You will never again listen to shitttty stock speakers...
So it depends, you want loud with sound quality or very very loud with low quality bass...
Personally , I would get bored with a sub pretty quick if its low quality sound. Sometimes you want to listen to that slow song which needs to be high quality. You can't just have bass in your ears all the time...
In short, if you want good quality, get good speakers for 10-12k. If you want only bass, no quality, get the boofers...