To rehash previous statements, marks of chaos are quite good in BFG:R, and I recommend implementing those full force, but keeping the additional add-ons from powers of chaos (such as the pink horror death blast, the khornite overload swarm, and so on and so forth). BFG:R also does sub-commanders right; to be useful, they must be cheap and not very customizable, for that is point of the fleet commander. As for CSM, as AndrewChristlieb pointed out, they don't gain as much benefit as IN vessels gain. What about +20-25 points?
As far as I know:
Mark of Nurgle: +1 hits is worth 10-15 points, an not being board-able is so situational that it is worth maybe 5-10pts? (Suck it, Nid and Khorne boarding fleets!)
Mark of Khorne: Doubling boarding value makes the tactic extremely effective, an the potential abuse makes it pretty good at 20 points
Mark Of Tzeench: Straight up price of a normal reroll. Functional but pointless, as any lord can purchase one at that price. Maybe at 15-20pts it would stand out as sorcerous advantage?
Mark Of Slanesh: -2 Leadership is significant penalty, but it does have a really short range. In BFG:R, they instead give incoming batteries a right column shift. Much better.
If we put all marks at 20pts and CSM at 20pts Termies at 10pts, the only problem is khorne boarding shenanigans extremely rape-tastic. A decked out slaughter would be 205 points, and its boarding value would be huge (18 or so if I am correct) at a very bargain price.