The largest connection, at any one time, is free. If you run just one connection, however small or large, there is no billing involved, and the account will remain alive.
As soon as a second connection is started, you will be asked to put in credit card information to continue this 2nd connection after a 14 day trial. The first connection remains running in either case.
Prices for Keller Williams are the list prices of PieSync but at half price. See https://www.piesync.com/pricing