For me, it's not just the cost of the splitter and painting it. If you're not colour coded, then a c/c'd splitter looks odd imo, so you'd have to factor in the additional cost of smoothing & spraying the skirts and rear bumper.
Whilst (imo) it may not work with all colours, the lack of paint work was part of the appeal of the rieger - it can go straight on and work with the standard exterior of the GTI