It really depends upon the extent of the rust. Mine started out looking to be a simple patch job but it quickly became apparent that I needed to do it right and weld in new panels. Make sure that you remove some panels and the front fenders to decide which of these replacement panels you need.
Dennis Carpenter sells the full floor panel for $600 - a bit pricey and the back of my floor was in good condition. I got mine for $75 per side (front only) from Northern Classic Truck here in Michigan. They are only just across town so I saved the shipping too.
Left side after floor removal
Right side at A pillar had the worst rust
Right side . . . you will need to trim the panels to fit your need.
And you may need to make some odd pieces to fit where the patch panel does not cover. Home Depot sells 1' or 2 ' sheet steel in 16 gauge. You may have to search through their stock as they usually only have the thinner 22 gauge in plentiful stock.
Dennis Carpenter full floor - $600 !
This is what I bought for $75 from Northern Classic Truck
If your floor is gone along with the bottom of the A pillars you can buy this assembly from NCT but it is $470. In hindsight I might have been better off getting a pair of these. Check your truck carefully to see how far the rust has gone.
A couple other tips:
AXRacer has done some great posts here on FTE on mig welding. You might want do a search on his name to find them.
EASB Easy-grind welding wire makes the finishing much easier.
Eastwood weld-through primer is good to use on the surfaces that will be blind after the job is complete.