If you have Guitar Hero branded wireless instruments you may be interested in some rechargeable GH batteries. They definitely work with the Guitar Hero: World Tour Guitars and Drums, the PS2 Kramer and the X360/PS3 Les Paul. The battery should last about 25 hours.
These are supposed to be £20 RRP but poundland got some in a few months ago and many a money-maker has slapped some up on eBay. Right now there are 12 listings with a buy-it-now price of £5 or less including delivery costs.
I managed to snap up six (2 guitars and drums with a spare for each) without paying more than £2.50 for any of them :). So if you are prepared to wait a little bit you can get a good deal from an auction listing. FYI: postage cost are approximately £1.50 if that helps you to determine the value you want to aim for.
Of course if you have a poundland near you it would be wise to check there first! I didn’t have that option.