Sent 4 batteries out to Primecell to be rebuilt on a Tuesday, and had them back the next Wednesday.  
I wasn't expecting to get them back until after Christmas.  They said their turnaround could be 3 weeks plus 2 weeks for a check to clear.
I haven't put them through their paces, yet, but they were junk before.  Drained after a minute of use.

Cost?  With the price of a good 18V LiPo set nowadays I can't see how it would be worth it unless it's super cheap.

I have done a few myself.

http://www.instructables.com/id/ReplacingUpgrading-Power-Tool-Batteries/

My family is invested in Dewalt 14.4 tools and have had some tools for 20 years.  A single battery now costs $80.  We have over a dozen batteries, and it just isn't cost effective to switch to a different voltage or brand.
And I was looking into rebuilding them myself, but the difference wasn't much more to just have them rebuilt with a warranty.

To rebuild the 14.4 was $38 and they come with higher amp batteries than stock.

http://www.primecell.com/pctools.htm

I did a couple of 16 year old 18 volt Milwaukees with sub C cells I got cheap on Ebay.  Been working great so far.