Whatever you need it to cost.
How much resources do you need to strip from the party? Some, Most, All, or More Than They Have?
The Castle, lands, titles, right to tax, etc should cost what you need it to cost to fit the flow of your game.
The DM has to ask themselves this question:
"What is my purpose in interrupting a long rest before it is 'completed'?"
Firstly, for me, I wouldn't care if the rest was interrupted at hour 7. I might not give the characters back everything for the fight, but after the fight? If the rest was...
Yeah, won;t change much at there tables. Just sometimes where their table is.
So the losers will have to hide in their homes to run their games, I can see this making some game stores nicer places to play at.
Yeah, not seeing the bug here.
OH NOES! The high level Wizard can keep up with his companions in the DPS department with out having to nova every few seconds! SKY IS FALLING!
In my opinion? Only some sort of Noble background. Everything else is present in the Fighter class.
For Style; either Archer, Defense, Duelist, Great Weapon Fighting, or Two-Weapon Fighting depending on what they see as their character's primary role previous to taking up adventuring...