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Depends on what mosfet you go for, some are cheap (I saw 2 AB ones on zero in for £25 and £35) and some can go up to £70 or so.
I make my own, because I can.
There are two types of mosfets. Mosfets that drop in (Like this one!) and plug in before you battery does, and mosfets that need to be installed and the gun needs to be rewired, and it's a lot of work for someone who doesn't know what they're doing.
The plug in mosfets are easy, and provide basic burst fire and I think one of them helps trigger contacts (delays the flow of current until the trigger contacts are together this stops the arcing you often get) but on the whole most don't do this. The mosfets you install always protect trigger contacts due to the way that they're wired in, and some have burst features (the ones I make dont, because I don't generally need/want a burst fire).
IIRC, you should expect an increase of rate of fire by about 20% with all mosfets, or that might be with just the installed ones... I can't remember.