"If i pay £10 admission , or even £8 should not the artiste ask a more inviting price for their CDs e.g £5 as they will already be getting a fee?"

No

Your £10 or even £8 goes to the organiser not the musicians.
Turn it round - If I buy a CD at £10 from the band, shouldn't the organiser give me a discount on my entry fee? - Let me know when you are going to try it, I'd like to be there

WYSIWYG - are you saying that you pirate cd's and justify it by suggesting you are helping the musicians? Might be better to take your friends along to the gig.