It costs quite a bit as you need to provide the rgb board/PPU (Google it, may be alternatives now?). Monstermug charged £50 for the mod I think. He's still on here. A good guy.
It's best to get one already done. I got one on here for about £130 fully boxed, 2 pads and Super Mario cart, RGB mod with toggle. Put a wanted add up. Ebay sellers generally ask for £200 I think?