there is definitely a leak in your AC. what happens is if the amount of gas/coolant in the system is low, it will cool initially but then the evaporator freezes and thats when it starts blowing warm air. switching off the AC for a while gives the ice time to melt and it should cool again but doing this is killing your compressor. get it pressure checked. leaks could be from anywhere, the condensor, evaporator pipes or joints. but as the gas leaks, so does the compressor oil so eventually you'll be running your compressor dry and burn the valves in it requiring changing the compressor as well. better sort it out soon before it becomes a multitude of problems rather than just one or two parts.the cost of repair depends on the parts to be changed and labour cost and that all depends on where you're based. can't advice on that. just don't go to dealers or they'll peel the skin off your back inch by inch with the bill