"The guy above" got his price from an RV dealer/service centre.
I should have clarified that the quoted cost was for the price of a 4 X 8 sheet (regardless of size needed, you have to pay for a full sheet at $400) and the $600 cost associated to the labour/installation. As I wanted the sheet placed under both the side and bottom trim and the job needed a sheet metal break (the front of my trailer is not flat like in the photos provided by ExxWhy), which I don't have, I figured I shouldn't tackle it myself.
Also, when I contemplated doing the work myself, other than the noted two reasons already given, I was also concerned about just driving in some screws to hold it as I was unsure of what type of material the screws would have been put into and whether they would have held firm (not sure whether they would have hit something solid or just hard foam etc). Perhaps ExxWhy could indicate how he determined what his screws were going into??
As it turns out, I may not be getting Line X anyway as, for the life of me, I can't remove the old adhesive (and I've tried everything) and he won't guarantee the application on an unclean surface.
Marv