Agreed; Mike Skipton (username Skipton01) has fitted OEM cruise to more A2s than I've had hot dinners. He's the man for this job.
On the original topic, it's a tough one to call, partly because there are no fixed prices for things. A decent set of HID headlights cost me about £100 and are a real benefit, however I've added much 'bigger' things to my car for less money. When I had SE seats but was looking for Sport seats, someone offered to swap with me. I only had to buy the wiring and connectors for the lumbar support mechanisms, meaning the upgrade cost me about £15.
Similarly, I retro-fitted rear electric windows into my car by swapping, meaning that upgrade was also cheap as chips... though I did trap myself in a world of pain for a while whilst I figured out how to do it.
Tom