It's all down to your insurance company at the end of the day. When I was with Kwik Fit they wanted £1000 for my 1.8 GL standard and that was the cheapest I could find at the time. When I put my BBS wheels on it they charged me £400! I did do this sadly. However 6 months down the line and I wasn't willing to pay £140 a month so I changed to Admiral on a 10 month bonus accelerator.
They charged me £960 with alloys, front and rear lights, audio, lowering and black badges and indicators. Which I thought was fair enough. Now I'm paying £650 after finishing that policy and with Adrian Flux who I'm all declared with but get like for like on my mods if I claim.
Ring up and get some quotes on what it'll cost if you do certain things, that's what I did, it gives you an idea then and what you can financially do and what you can't do.