Cliff Lee is gone. That ship has sailed. And in his defense, it wasn't about the money. Cliff enjoyed Philadelphia and took less cash to go there.
As for the Rangers, their next move should not be rushed. The worst thing to do now is to panic and spend money somewhere else just to "do something". I wouldn't overpay for Zach Greinke (KC wants the farm). I wouldn't overpay for Matt Garza, or anyone else that will require a ton of prospects.
If you remember, the Rangers shot to the top of the AL West division without Cliff Lee. They would have very likely won the division without him. I see nothing wrong with going in with the rotation as presently constructed and look to make another deadline deal to put the team over the top if they are contenders again.
There are other areas that they can improve the team without acquiring an "ace". They need a DH. They could use a defensive 3rd baseman. And you can always use bullpen help. Look for the "smaller" deal to improve the team and again, be ready to strike at the trade deadline.