Wow, those are open ended questions!
No way to know how much damage is done till it's opened up. You could ask them to remove the inside surround to the Fantastic Fan and take a look in, but even that won't tell you the whole story.
Cost of repair is totally dependent on the amount of damage and how they can go about repairing it.
I've walked away from a couple rigs that were very nice otherwise just due to a rook leak. One I looked then called them and said I wanted to come back and look closer at the tear in the roofing. They bought some repair stuff and covered the area before I got their. Took one look and left. Just didn't want to play that game!
2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft
Charter Good Sam Lifetime Member, FMCA, SKP
RV'ing since 1957, NRA Benefactor Life, towing '05 Odyssey