you need to do a hell of a lot of welding to write of the cost of it, all it does is stop or slow down the surge whebn you pull the trigger, i find it quite handy to have this for tacking stuff together as it seems to produce a better weld straight away.
Here is a link with some explanation and here is an online seller - no idea what they are like though - sorry. From the description of how they work I have to admit to being a bit sceptical and, moreover, if they were that good wouldn’t they be more popular?
I have been using one of these gas savers for a year or two.
I'm not too sure whether it has made a difference but at £30 ish i thought it was worth giving it a go.