You have to take into account any future setup then. Center speakers need to be matched in timbre and tone to the speakers around them or you're not going to get a wall of sound effect from your front 3 speakers, you're going to get a stereo pair and some different sounding speaker in the center.
So, if you want to do this piece by piece over time I'd look into what mains you'd like in the future and go with the center channel that matches them.
If you're looking for a whole new system I'd recommend
Axiom Audio; going by your price for the center they're in your range.