ive just discovered the first bits of rust to be worried about on my van! a bit dis-heartening but obviously inevitable

directly above each front wheel, if i look upwards inside the wheel arch, on a flat panel of metal that is parallel to the ground, what looks like a seam (running from van front to van rear) is coming away with some crunchy looking rust about 15cm long. its well away from any suspension or (to me) anything structural, but looks like it needs attention!
its not the footwell/step that seems to get some common rust/water collector. it cant be seen from the top side if i pull up the cab rubber floor mat at the sides, but it does look pretty grungy under there.
heres a picture (not my van) but im talking about the underside beneath the can of WD40 on this image:
i dont know anything about welding so i wont be doing it! it looks like a square welded patch of about 20cm by 10cm would cover it once it was cleaned up (on both sides). can anyone take a stab at what that kind of welding might cost to get done??? (its a flat section, and the new piece would just be a sqaure id think).
thanks!
john.