If you have coolant coming out of this drain, you definitely have a heater core problem.
The heater core itself is not very expensive. Maybe around $50. It is the labor that is going to be expensive.
I swear, every time I changed one of these, they took a heater core and built a car around it.
There is a lot of dis-assembly required under the dash in very cramped quarters. Some installations also require that the engine is disconnected from the motor mounts and rocked forward to access the parts of the heater located on the firewall.
I would figure on at least $250 for the labor. Call around and get some estimates from some reputable local mechanics.