Forge DV 007 is a dump valve (hence the DV)
It acts as a more efficient replacement for the standard wastegate system (which vents excess boost pressure during throttle lift off).
Basically, when you lift off the throttle on a turbo charged car (to change gear for example) you need to vent the pressure side of the system to allow the turbo to be able to spool up again when you get back on the power.
If your engine is running more boost than standard the wastegate system will not be capable of venting all the excess pressure, so the turbo will be battling to spool up against the back pressure retained within the system, and you'll have Turbo lag (as well as a prematurely knackered turbo).
Cost? from ?100 I guess, not running one, so I can't say, I'm sure one of the petrol boys will chip in with the cost.
