You can RAF yourself 1-85 in 3 days of real time about 24 hours or less played time 8hrs per day. To buy a new account currently that is RAF read you are spending :
Your Main Account or a new account: $10 (battlechest promo links around or buy mop and get the $10 battlechest)
Second account: $10 and gets 30 days free game time
So for $20 you are setup 1-85 for 3 months with 300% increased leveling speed. Within a week(7days) you can get 5-6 85's if you grind hard. Questing 1-15 then dungeons as tank/dps healer/dps combos
If you already have MOP on your main account you only need to invest $10 for the second account to get a 85 in 2-3 days. If you dont then you need to spend another $20 for MOP upgrade and hit 90 in 2-3 days after 85.
So total at most your looking at a potential $40 for Battlechest,MOP + battlechest 2nd account. or $10 at the least for your 2nd account to get battlechest cata and RAF level.
If blizzard charges more than $40 to boost your 90 then it would make no sense and everyone would just do the method above to save money (select few who are financially well off or dont play hardcore will just buy the 90 anyway regardless of price).
If the 90 boost is less than $40 blizzard makes a killing only for those who dont have MOP already. If someone has MOP they only need to invest $10 and can do 85 in 3 days saving themself whatever blizzard would charge for a 90. Level 90 gear I'm guessing will be around 496 gear level since timeless isle is 496 catch up and thats what this boost will do in a sense.
Not including the WoD 90 you get just the ones you can buy from the store in future.
Now the question remains if tons of people do RAF the active sub numbers will be padded data (as we will see in Q1 2014 since unconfirmed sources say almost 500k bots were banned)