Jesus Christ.
If your budget is so tight that you can't come off some money for headers, how EXACTLY are you going to finance a headswap???? What is your contingency plan if you or your shop mis-times the cams and you bend some valves? What would you do if a tech accidentally dropped a machine screw down your intake valves and trashed a cylinder?
Take it from someone else if not from me. This is not an area of your life where you meet lots of people who took the cheapest possible way out and lived happily ever after. The cheapest parts are often the ****tiest parts. If you really want to get serious, do it once and do it right. The higher initial investment typically has a lower life-cycle cost because maintenance and rework is not as big of an issue.
I never understand why people can't find enough time or money to do a job right, but they have enough to do a job over again.
BTW- to answer your original post... You don't want headers to rust for a reason. The tubes are much thinner than factory manifolds. If they rust through (and bare metal or poorly treated ones DO) then you get to pull the ruined headers and pay for new ones. Again, doing the job once right is cheaper and easier than doing it twice badly.
EDIT- I always sound like a hardass in these posts. I'm not trying to be a jerk, but sometimes it takes patience to get what you want. I've been driving Mustangs for 6 years, and I just got LTs on my Cobra (which took 4 years to get) this past month.