Here's the way I see it:
Hatcher got paid a lot of money after dominating at the 3T in a 4-3 and now he's moving positions again to be a majorly run stopping DL. They aren't really getting his true value this way. He's also 31 or 32. If he doesn't work out, they have to keep him unless they wanna take cap hits.
Melton got paid minimal money for a one year contract. The Cowboys essentially have his "Byrd rights" so if he balls out and gets back to form all they have to do is keep him, they don't have to go and try to outbid for him or risk losing him because he won't be a FA. Oh and if he does get back to form, you have a dominant 3T in his prime for market value or cheaper and will be thrilled to pay him. If he doesn't pan out, he likely didn't meet incentives which keeps his salary down and Dallas simply need cut him and no harm done.
I like(d) Hatch and I'm glad he got paid, he worked hard for us and did well but we got younger and much more potential with no financial risks. Overpaying old guys is how we got into this mess in the first place.