2006 BMW 750 LI having to keep coolant in it a couple times this past week. I'm sure it's leaking but What could be causing the problem and is it costly?

Where is the leak?

These are the common problems, and more affordable to fix. You could have a bad water pump seal. The overflow nut could have cracked. You could have a simple leak in a line. When you have put fluid back in the radiator, did you make sure to have your heater turned all the way up so that thermostat opens up? If you did not do that, there is probably air in the system which will require many additional refills. Try this and see if there are any leaks.

I would recommend doing a pressure test to the cooling system to find the leak. Basically you hook a special tool up to the system and pump pressure into the cooling system circuit and watch for any leaks or pressure drops. But if you dont have the tool just let the vehicle get to operating temperature then shut if off and look for signs of a leak the cooling system creates and maintains its own pressure. Your particular engine the N62 can have coolant leaks in many places. Hope this helps let us know how it goes.

Since I have the same car (an '06 750i) my recommendation would be to take it to a trusted mechanic and let them do the pressure test. If it is not diagnosed correctly AND repaired correctly, you risk a much more costly repair down the road. Not for nothing, but this is not a GM or Chrysler you are driving. These BMW's are well built cars, but when they break down they are very expensive to fix. To just keep adding coolant is a quick, cheap option but it will not fix the problem. Getting it checked out and fixing what may be only a minor issue before it gets worse is the best wasy to go. Good luck and keep us posted!