Puddles on the Floor after using the AC. on 2001 Mitsubishi Galant

When the AC is on, water slowly starts to build on the floor of the passenger side and back seat floor behind the front passenger. I have two big puddles in my car now. The driver side is completely dry. The problem was on the driver side and then it switched to the passenger side. What is the problem and is it an expensive fix?

I agree with Zee, the heater core/evaporator housing has a "drain tube" it it get clogged or dis engaged from the firewall water leaks on to the passenger floor. If it were the heater core that was leaking steam would come out the vents or you would smell a slightly sweet smell and experience coolant loss.
2 more answers , 1 more comment
Looks like the evaporator drain hose is clogged and condensation water can't drain out.
Ask your mechanic to check the drain.

my car has the same problem. where can i find the drain hose?
sound,s like your heater core is leaking, that,s not good,