The simple fact is that some CAMO's hide the cost within the labour rate, others are up front about the cost.
My guess is that under BCAR all the paperwork costs were hidden in the Labour rate as you would be dealing with one company.
As subpart G requires a seperate corperate identity (with all the costs of another company) the subpart F & G will require seperate billing, The subpart F & G are seperate approvals that are audited ( and paid for) seperatly and so you get an invoice from both companys.
The fact of the matter is that the subpart F bill should be a little smaller however as most of the parts for aircraft are sorced in US$ (or Euro for Robin) I would guess that the bills have gone up due to the weekness of the GB£.
The only true indicator of a ripp off is if th Subpart F labour hours have not fallen a little.
Bose X I hope that this is logical enough for you however I would like to see you try to run a CAMO, I am sure that you would soon change your tune.