The cost depends on the following:
The condition of the Compressor unit, the fan coil units &
the condition of the ducting system. It's best u get a
reliable air cond repairman & ideally from a recommendation of
a friend. Additionally u should get a free quotation without
any obligation.
Psi is pound per sq inch pressure -this is an indication of the
amt of gas in the compressor unit & the repairman will use a
gauge to check for the correct amt in your air cond.
Hope the above info will be of some help & all the best 2
u.