DeWalt Tools, Replacement Battery question?

DeWalt Tools, Replacement Battery question?

I'v got six lithium- ion, battery powered, DeWalt tools, and only one battery between them all.
Time for me to get at least one more.
Boy, come to find out these official DeWalt replacement batteries cost $100.00 dollars, and more these days.
I would like to know, find out, if generic batteries that you see advertised on ebay, at cheaper costs are worth it to purchase, and work just as well?
Anyone care to comment what batteries you use for your tools?
Thank's a lot.
Jim V.
hemi71x