Wow. That's a lot of rot!
Speaking as a metal fabricator and automotive restorer / hobbyist...there is a significant work load ahead in an area that is mostly structural. The price you state isn't out of line. In fact it could escalate. If you go "CHEAP", that's just what you are going to get and you won't be pleased with the results.
Replacement metal sections from a 03' to fit a 99'? That's a pretty big leap in model years. I'd confirm / measure / compare the donor portions prior to committing to purchase if I were you.
I'm additionally concerned w/what you CAN'T see until one is cutting and hunting for decent metal for which to join to.
Part of me says unless the REST of the vehicle is in
outstanding condition / low mileage, I'd unload this one. This may not be what you want to hear though.