I've seen the replacement steel for the problematic area behind the front tires that rusts out. Mine have holes in them.
Which kind have you used?

What goes into this proceedure of having them welded on? Is filler used also? And how much is reasonable to have this done if I take the fenders off myself?

Same as replacing any other panel. Im a noob and knwo that its just about getting (or fabbing) the panel, cutting the old out, and slowingly welding in the old. foloow it with grinding, preping and painting... Its just steel and the more time you spend with it the better. Start on practice panels. Check my newest threads where ive shown pics of noob done panels.

i purchased one from OPGI a few years back. it was a good piece. had a shop install it same time got car painted. it should not use much filler so long its welded correctly. i could not tell u how much. it depends if u want it finished as the color of the car. also, what needs to be painted. the whole fender or just that bottom portion. i would get quotes from at least 2 shops. ask how much with fender on/off car. might not be much difference and it will save u lots of time. more importantly, take it to a shop that does this type of work -not a regular production body shop. actually, for something like this, i would get a quote (just for the sake of it) from a regular body shop too.