Edit - I posted all this assuming you were gonna buy the adapter "kit", and not just the $40 adapter. I'll leave it up anyway for other people since sales are posted, and someone else might be in a similar situation..
just putting this out there, but for the cost of the batteries, adapter, and charger, - you are not far away from a new drill kit, with batteries and charger, or maybe even a drill impact combo kit on sale. A new sawzall bare tool would be under $100 I think.
The adapter kit you're looking at is $129 with the small batteries..
This 4 tool kit is on sale, $100 off, for $249. You can sell what you have on eBay to put towards it. Even sold for $40 for each tool, you won't be far away from the money you'd spend out of pocket anyway - but with new tools and an impact driver and flashlight too.
http://www.homedepot.com/p/DEWALT-2...-Cordless-Combo-Kit-4-Tool-DCK420D2/205089392
MOST people use the opportunity to upgrade into a new system. IMO, The adapter kit only makes sense for people that have a large collection of older Dewalt tools. I was in the same boat, and upgraded to the Milwaukee m18 system.
Just making the point, you have options here. If there is ever gonna be a time to upgrade tools and get the most for your money spent, this is it.
A bare tool 20v Dewalt saw should be cheaper than the adapter kit if you already have 20v stuff and batteries. Just doesn't seem to make sense to lay out the money on the adapter kit to keep 1 tool going that you can replace for less money. There may be more to your situation, but just off what you posted, buying new seems like more for your money.
Yea, the bare tool saw is only $99 right now.
http://www.homedepot.com/p/DEWALT-2...Reciprocating-Saw-Tool-Only-DCS380B/203164237
Another option, on sale for only $20 more than your adapter kit - you can get the new 20v saw, with charger, 5.0 battery, and soft case.. your getting less batteries, but more battery power with the 5.0, so another amp hour of runtime if the adapter set comes with 2.0 batteries. This would actually save you money if you sold the old saw.
http://www.homedepot.com/p/DEWALT-2...-XR-Battery-Starter-Kit-DCB205CK380/301943595