Just did a real quick price check for Kobalt 80v 5 Ah batts. Did not find a single one for less than $200 total shipped price.
I think we paid around $335 total, for our KCS 4080-06 saw, batt, & charger package.
That means that you get the batt & charger for about $125 more than the price of just a batt. That's a no-brainer for us. But, from what we've seen, we'll never need 2 batts for this saw.
Now, I did notice some used 80v 2 Ah batts, for less than $100. I think one is only $50. So, I suppose you could buy up several cheap 2 Ah batts & have plenty of power for many hours.
Here's a new one for about $82 total shipped price.
https://www.ebay.com/itm/Kobalt-KB28...4AAOSwt0Ffa3Pc
Here's a 2.5 Ah batt PLUS charger for just under $100 shipped. Not a bad deal.
https://www.ebay.com/itm/Brand-New-K...75.c100623.m-1