gary
the question you are asking is like
how long is a piece of string
are you talking outer sills inner sills skin sills etc need more info
need pictures
it,s normally the back end of the sills outer and inner that rot most inner sill at the front end
still really need to see the car or pictures normally what looks ok when see it after you chop the sill off you find a lot more problems normally at the ends as i said also they rot near to the rear axle beam mounts on festers
Theres no such thing as a firm quote/fixed price for replacing car sills to many variables, as to how much work you will un-cover when removing the outer panel.
It all depends on how far the tin worm has spread.
When I done sill work 20+ years ago, I used to give 3 prices,
Outer and inner sill replacement (not un-common).......100.00 per side.
Outer sills plus a couple of small inner sill patches patches (most common).....50.00 per side
Just the outer sills and shut panel (very rare)......30.00 per side.
I always told the owner the most expensive to start with that way it gets better not worse, I did a couple of jobs very early on and lost money hence high to low pricing.
Generally labour prices have doubled since then, but I don't have any idea what the current price for sill work would be.
Phone some local body shops and ask for a price after your car failed its MOT, for a starting point.....
Just in case you didnt know it. This thread gives an idea of the level complexity, and thats only a three door!. (sill pics start about third of the way down)
I'm not saying its difficult, as it isn't. but if you blow through that very thin modern car sill, or get KO'd because some ass at the MOT place insists on spot welds and not plugs, it can be an expensive do. Get an experienced mate or family member to oversee if you are unsure about how to go about it.
I would say a garage would not charge less than £300 at the absolute minimum, and maybe twice that. Depending on whether its a straight outer replacement only, or whether the inner sills/door post, etc need fettling.
If you, are at all good with your hands and can wait for the warm weather (Hah!) and have a go yourself. With Fiestas, parts are cheap, and there are always lots of guides to refer to, and Forums to ask like minded people. I wouldnt attempt it till my welding/plug welding skills were reasonable (unless you have some spot weld drills and can borrow a hand held spot welder yourself, would make it a LOT easier)
Had to repair holes in my two sills last summer, and that took me all week from start to finish. But then that was me doing a job for myself on my own vehicle and I wanted it perfect.