Let's say debuchy is on 60k and sagna wanted 130k. That's 10,920,000 difference over 3 years. So given that those 3 years would be age 28-31 for debuchy and 31-34 for sagna, the deal would make sense.
I know that's guesswork but it just shows that there's no huge difference, debuchy is younger though and not demanding things that he isnt worth.