Most of them are in the $200-300 range... just do a quick eBay search. You can get the OEM BMW Performance one for about $230. IIRC, there's another one on eBay in the $150ish range.
But FWIW, a lot of people say that a strut tower brace should be the last suspension mod that you do, after the rear subframe bushings, the front control arms, etc.
2008 135i Montego Blue sport pkg, premium pkg, Logic 7 sound, iDrive/Nav
Mods completed: JB4+ & FBO || LSD || BMW-P suspension + F/R sway bars || 18" Breyton GTS-R || M3 front bushings || Vogtland camber plates || BMW-P rotors + Stoptech pads |||| Acquired, not installed: BMS WW Meth kit || Logic HVI intake || Mishimoto OCC |||| Shopping for: Summer tires