well the kit is cheap, like less than 300. not sure, i have a quote from Logan themselves. what's expensive are the heads and what not. the 05+ Intake is cheap from Blue Oval but the heads are killer. can only find them ported. besides that you'll need a couple of other things. it's a pretty expensive swap i'd say.
powerwise, someone posted a video of someone that did it on a PI GT and he went from stock to 350 RWHP or something like that with the swap and exhaust i think. not sure, search for the video
some money? a lot of money. the heads are $1000 or more each. intake is cheapest, like 90 i think on Blue Oval. then the kit, then some other parts, then the labor by a highly experienced Mustang shop (that's what Logan said). that's a huge bill