JMHO here, but often when one is paying “part” of a bill like this in actuality one is actually paying all or most of it. Why? For this reason.
If Subaru covers the cost of head gasket repair under a legitimate factory or good-will warranty this means the dealer is working under what SOA (Subaru of America) allows and will not receive one dime more than that.
This means that parts/labor the dealer is allowed to be reimbursed for is drastically cut.
Parts? Cost + 25%. Not one penny more.
Labor rate? Discounted often 15-20% underneatht the normal shop hourly rate.
Labor time allowed for the repair? This is where Subaru is notoriously chintzy. The labor rate will often be about 1/3 of what the book calls for; IF the shop is lucky.
When adding all of this up odds are the total parts/labor (600 as mentioned) may not exceed that and may even be much less than 600. Subaru of America and the dealer may not be making much, if anything, on the repair but then again they’re not losing money on it. It serves as good PR though.