Players you can use only for one year eat up your money. FA that cost the wage value to sign practically use up 2x wage in one single season as they retire, which makes them expansive to use.
Young player who can be used for 2 years will be at a higher market value than they were 2 years ago. And for that the same money paid run for 2 years at least, therefore the cost to run is halved (assume you don't renew contract and sell him on third year).
Even if you renew the contract you pay the one time bonus that is good for 2 years, that is 33% cheaper than a retiring free agent.
At a money shortage, you should keep your 18y and other young players whenever possible, and only consider selling if their contract is about to expire. It can be unintrutive to keep a young player team at money shortage, but this is the most efficient way to gain money. Your wealth lies on their market value and when you sell them at 21 you will end up with a bigger profit margin, even more if you managed to sell him through negotiation offer.
Your balance should be positive if you keep a 22 player team, doing 2 games a day (1 league and friendy if no cup nor CL)
Invest properly so that your wealth won't slip away. Rebuilding your team may be difficult, but should be possible without token spending.