Some dents can simply be pulled out with a suction device, others need more attention.
If panel replacement is necessary, then it depends on which panel it is that has the damage.
Side panels are vey long, and therefore somewhat more expensive than the smaller panels. In most cases you do not need to disturb the interior, as far as I know. I was quoted $ 1500
.00 to replace one of my side panels, which runs the entire length of the trailer on the street side.
I would not like having extra seams, and just replace the damaged area, butthat is a possibility also.
Make sure the dent is not where there is a rib behind it. If the rib is bent, then you should be able to see it in the interior. I imagine this would be a lot more work and expens than a dent between the ribs.
yoiu can tell where there is a rib by the vertical rivet lines.