It is true that tool (and other) manufacturers price batteries prohibitively. Also, each of them tends to include some feature that makes their pack unique so they alone can hope for custom for replacements. However, if it fits and the voltage is the same then the choice is yours to buy it.
Often you can 'crack' the battery pack in a vice, or it might even be held closed by screws. If your battery pack went bad suddenly it might be just one cell. The cells are available from good battery/electronic shops. This gets you around the problem of fitting into your drill and is much cheaper than buying a brand pack.
The cheaper drills always outlast the battery packs. If you replace the fading cells with good quality cells you'll have a cheapish drill that lasts for a long time.