Is it possible to take a bus from Lima to Cusco ? How much might it cost ?

I think the real "cost" here is the time and discomfort. The direct trip from Lima to Cusco takes about 20 hours and involves travel on some twisting, turning mountain roads. Other services use a less direct route that requires you to change buses in Arequipa.

Yes, it's easily possible. It will take a while, but Cruz del Sure (http://www.cruzdelsur.com.pe/inicio_2.php) will do it for you. The bus station in Miraflores has several companies, pop along there and check you the various companies.

You can also fly with TACA (taca.com), although that's less fun

We took busses from Lima to Arequipa, and then from Arequipa to Cuzco, but altogether that oils make about a 20 hour trip, which is basically the route you would take. We opted to fly from Cuzco to Lima and paid $130 pp on Star Peru. No complaints about the airline or the flight, and it only took about an hour.

(forgive the auto correct error in my post above!)

If you have time you can go via Arequipa and break the trip. To save money over LAN, try the alternative airlines...StarPeru, Peruvian Airlines or TACA.