Ric sorry but i agree with Gizze £3k for an audio upgrade sounds bollocks unless they are looking to install a new stereo setup with new wiring with new equaliser etc. But if it is just a speaker upgrade, amp etc then it is bollocks.
I have had an audio upgrade done to my BMW E60 and it sounds awesome, here is what i had done:
1. Hertz ESK163.3 components which included 6.5 subwoofers under seats, 4"door components and tweeters.- £200
2. Audison SRX5 multi channel amp - £150 (ebay)
3. 2 12" JL subwoofers in boot. £250
4. Factory stereo and sound still controlled by i-drive.
5. Cables etc and all fitted
Total cost was no where near £3k and sounds awesome, it cost me £700.
Talk to the guys at LM Audio (Google them)(speak to Lee and say Mukul recommended you) and if local to you allow them to do the install or give Gizze from this forum a shout, he knows his stuff but don't pay £3k to this audio installer, belive me it is a rip off. Out of curiosity which audio installer quoted u £3k.