I must admit that the Mastermix type compilations are very expensive for single CD's, I suppose its down to who they are aiming the products at - i.e specialist market demands specialist prices

. Until they get some serious competition it will always be the same.
£25 per CD is a lot to pay, especially when you are just starting out. It's worthwhile remembering that if its just the non mixed bog standard music tracks that you are after, then Mastermix is not unique in its product.
For instance, compilation CD's containing music are always available in many variations and from all era's. I've found double CD's costing £14.99 or £19.99 from HMV which have more than covered 80% of the tracks contained on 2x Mastermix compilations!. Do the maths - 2x £25 Singles or 1x 19.99??

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Talk to your local independant music shop. If its a sole trader they are far more open to negotiating mutliple compilation purchases (especially £500 worth!) than high street franchises. If you buy on a regular basis then don't be scared to ask for a discount. Retailers margins on Cd's are low, but you may be able to get a small discount if you go in there every week, or even the odd £2.99 freebie from their bargain bin

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You don't have to pay for DJ Only cd's to get the majority of party classics either. Black Lace "Party" Cd's go for around £1.99 nowadays, other CD Compilations containing party tracks are available for between £5.99 and £9.99 with compilations offering double and treble CD Box sets for as little as £20. Also check out the box sets from Time Life and similar. Their "Rock", "80's", "70's" "60's" box sets offering excellent value for money for a fraction of their Mastermix Counterparts.