How much would it cost to replace the internal battery on an HP laptop?

Depends if its soldered or just clips in.there usually just the disk type you can get from poundland and if it is a clip in one it will cost you a pound .don't know about the soldered ones

I believe all HP laptops have a clip-in battery and that they are easy access.

What model is it?

If it's one of these you need;

http://www.amazon.co.uk/Genuine-Laptop-Battery-593553-001-Original/dp/B00E9S0UKE

Then I have one here you can have. It was a spare battery for my spare laptop and I've never used it nor am I likely to.

Check your model and if it helps you can collect whenever you want.

Thanks for the offer Slinky. It's G61 though, which isn't on the list.