There are 2 screens in the touch screen area of the ds, the touch screen part can be bought for as little as £10 in the uk off ebay. this does not include the lcd screen, if that needs replaced, add another £15-£30. Now I've never replaced a DS, but i have replaced a PSP screen, which was quite easy.
The replacement screen should come with instructions and a screwdriver, have a look on ebay.
I have never changed a Nintendo DS touch screen, but I have changed a gameboy screen. I used an online guide, and this is where you can find one to help you:
Is it DS or DS Lite? The Lite touch screen module is more expensive than the regular one.
I just replaced the one on my DS Lite a few months ago after dropping it. If you need help replacing it, PM me.
BTW, if you would rather go through a store instead of through eBay, try http://www.emvee.ca/ That's where I got my replacement module last time. Their prices were very competitive to eBay's prices and provided free shipping, so I went with them instead of getting myself through the risks of eBay.
