Depends on the player. Some are worth it. Have signed Riquelme several times for example.
Basically if you are desperate to get money for them in the end, don't sign them.
Their Value doesnt really matter once in our team, Scholes, Giggs and Beckham are all worth next to ****** all, but can do a job. And Totti is pretty awesome for 33... Also good for mentoring younger players...