Bro the guy is trying to rip you off!
If the pipe is leaking, it's not that expensive to replace. But if water is leaking from the evaporator body, that might take longer to fix but still it won't cost you 1100 bucks!
Ask him to identify the exact source of the leakage then only can we determine how much it would cost to get it fixed.
Find another A/C mechanic. There are lots of guys on Pakwheels from Islamabad who could recommend a reliable place to get it fixed.