Stage 1 is typically. boost increase - to 1 bar, remapped ecu, to cope with the extra fuelling demands.oil cooler, Exhaust-usually right through and an induction kit. Optional extras are iridium spark plugs, hard interccoler pipes, boost controller, if you are lucky they may have done the intercooler.
Usually you'll see 350-420 bhp depending on if the IC has been done.
It may be worth getting it on a dyno just to check there are no hiodden surprises waiting for you.
front pipes = 285 ish
DE cat pipe = 50
cat back(tailpipe) = up tp 600.
About 15 each
So without an intercooler or hard pipes you should be looking at about 2-3k
You could scource some of these parts second hand to save some cash.If you do the intercooler then it would be a good idea to get the boost controller as well.
You'll have to add VAT as well !! and fitting.
I may be wrong on these prices as I am not in the trade. they should give you a good idea though.
hope this helps.