my thoughts on this. the solution all lies in what part is leaking. if it is an end tank, the seal around the end tank then a quick fix additive wont work. It could also just be the o ring around the drain and that is replaceable for less than $1. If there is a hole in the tubes, i would use a product like thymeclock suggests, the old bars leak or pepper are decent. I have had luck with these. I would not use some of the equally priced epoxy alternatives. It could even turn out to be a "permanent" repair.



I agree though that $400 in parts is way high. A rad can be bought off ebay for $60 shipped, $15 max for coolant, and even if you replace the hose clamps. Unless they already looked at it and determined your auto transmission cooler lines will need replaced (unlikely) or something, this is a $100 in parts job.

Do it yourself save money, rather easy repair.