the extra cost isn't cause its a tighter bore, its cause its better. Honestly, systema TB suck. ive used them, and they are only marginally better than the stock barrel IMHO.
Prometheus are more expensive because of the materials used, the finish that is applied, and because of the tight tolerances of the barrels (the same can be said for their internal parts as well)
dont buy systema TBs. Get prometheus. You will love it and i promise you, you will have the best performing TB around.
The only other brand that comes near prometheus, is KM (at least from what i have read). Madbull is decent, but apparently their barrels are thin walled. Ive handled Madbul barrels, and it seems like there is some truth to that statement. Prometheus or KM all the way.