Toss and replace.  The cost of a new heater core vs repair costs do not
make repair economical.

Most are labor intensive, best to put in new. You'll probably find the
cost of repair as opposed to new pretty close anyhow. Some are plastic
& aluminum & unable to be repaired in most cases. Good luck.

You'll find it difficult to find anyone who is even willing to repair rad
and heater cores these days.

Toss and replace is the only real practical option.

considering you usually have to dismantle 1/2 the car... I'd hate to
have to do it twice after the repair starts to leak...especially if
you're paying someone by the hour to R&R it twice.

You are aware why they call it a heater CORE, right?  It's the CORE of
your car - they start on the assembly line with it and build the rest of
the car around it.  (ok, so I'm kidding, but it sure feels that way...)

Ray

In my experience, it is difficult to repair them so they don't seep
coolant.  Since it's usually such a PITA to get them out to begin with,
it's a false economy to try and fix them.

The one for my car is $13 US.  A copper core is $23.
OEM's are aluminum.  It would be very difficult to repair
paper thin aluminum eaten out by electrolysis.
Several hundred dollars I would imagine.

Depends on the car and how mechanical you are. For instance,
on my Fiero it takes 30-40 minutes total to replace the heater
core, and that's if you take a break for lunch. I can have
the core out of the car in under 15 minutes. The cores in
my Fieros are brass, and my buddy at the radiator shop only
charges $10 to resolder the end tank (where they tend to leak)
so in my case it's much easier and cheaper to repair than to
replace.

My Phoenix, on the other hand.....

JazzMan

I concur with the majority of the other posts, unless you drive
something rare and exotic where the heater core is made of unobtanium,
you shouldn't even think about having it repaired if it's leaking.

good luck,

nate