if you just want a gauge that will tell youw hat's going on then go cheap and nasty
if you want acuracy and life expectancy then you need to spend some money

just find a vac pipe and plumb it in, but there are a few things you need to consider before you cut and dodge
the amount of boost the turbo provides is different to the amount of boost the engine sees, so you need to decide which you want to measure

at one point i had 3 boost gauges on my car, one each from the turbo's and one in the plenum and there was a good few lb's of boost being lost and one turbo was blowing harder than the other, even though the end reslut was an equal amount of boost at the inlet

on the other hand, i was fortunate enough to have an actual adapter that i could unscrew from the plemun and put a fitting into

also read up on stu's words where he tells you how to make sure you don't have any boost leaks or how to minimise the amount you lose through the system