When I wore out a battery and found out how expensive replacements were I just bought individual cells and rebuilt the battery pack myself. I don't know if it is still the case but my 14.4v DeWalt packs are screwed together rather than glued so I can easily disassemble and rebuild.
I lucked out and have a handy dad (he is an elevator mechanic). Those replacements dewalt battery packs are spendy, more than half the cost of some of their drills. As a result he just buys a new drill when they start to wear out. My first dewalt had like 6 25% battery packs that came with it :)
