As a club you don't even pay a dime in signing of a player that's a free agent to any club you just agree personal terms with such a player and he signs the contract to become your player. It's easier to get a free agent player unlike the other one that you need to agree with the player on personal terms before proceeding to meet with the club in question that still has a contract with the said player. I agree with you here, getting players for free right now is the best bet for barca in their current situation right now.
Getting a player for free is good but hope you know that you can't get a player that's in their prime for free. Unless you want to be doing like real Madrid club who usually go behind the club and talk to players they are admiring not to renew contracts with their clubs. Most clubs don't allow their top players to leave for free. The players that you will likely see for free are the ones in their early and late 30s, or the players that not performing very well. Barcelona can't depend on free transfers if they want to be competing with the best in Europe that are splashing big money to get quality players.