Full bumper and diffuser, including delivery fitting and painting you're looking at about £1000 - £1200
RT Performance took a copy of my bumper and I've seen 2 or 3 of those fitted. They're not 100% perfect but pretty good.
I believe the RT version all in maybe £700.
There are cheaper ways of shaping the rear end but I'm afraid Chargespeed is one of the most expensive options.
I was like you, ticked all the boxes and saved up.