As long as you buy the right unit, it's an absolute doddle.
Go to your local Halfords and get the removal tools (or Renault might do them too.) It's possible to improvise, but the proper tools only cost a couple of quid and make the job so much easier.
Before you start, make sure that you have the code for the new player and know how to enter it. Normally, you twiddle the wheel on the back of the remote to set the number and press the button on the bottom of the remote to advance to the next digit. A long press of this button confirms the code, and provided you have the code correct, activates the unit.
Insert the tools into the slots in the front of the player. Pull, and the whole thing comes out. Disconnect all the plugs from the back, reconnect them to the new player, and shove it back in the hole.
If you're unlucky, the wires will ruck up behind the new player and it'll refuse to go back in. If it happens, take it out again, adjust the position of the wires, then try again. Persevere, but don't use force - it took me about five goes to get the player to go back into my old Laguna the other day.
As for the price - see exactly which unit the dealer wants to sell you, then see if you can find an identical one on
for a lot less money. There are plenty of them about.
An alternative would be to buy a compatible 6 disc CD changer. This way you get the versatility of being able to play both CDs and cassettes (the cassette player is handy for audiobooks if you ever listen to them.) Installation is a bit more involved - you have to find somewhere to put the changer and then run a cable to the head unit, but the end result is well worth it.
Edit: In fact, why not save a bomb and have the joy of an MP3 changer? Have a look at this link (kindly provided by L2O on another thread.)