WBA - Three percent (3%) of the purse of each boxer with a minimum of $1,500 for the challenger and $3,500 for the champion, or the maxium of $150,000.

WBO - "Three percent (3%) of their purses with a minimum of $1,000.00 and a maximum of $90,000.00 per boxer."

WBC - "Three percent (3.0%) of all gross amounts received by the boxers as compensation for a bout... On gross compensation up to US$100,000 a minimum of US$3,000.00 shall be due from the champion, and US$1,000.00 shall be due from the challenger. In the event of a vacant title, a minimum of US$2,000.00 shall be due from each boxer."

IBF - "Three percent (3%) of the purse of each boxer with a minimum of $1,000 for the challenger and $1,500 for the champion"

So lets look at it this way if fighter A has all 4 belts and fighter B is challenging for all four and they decide to pay all of the sanctioning fees....

Assume a total purse of $5,000,000 for each fighter
This is what they pay out in fees

Fighter A (champion) and Fighter B (challenger)
Each pay...........
WBA - $150,000
WBO - $90,000
WBC - $150,000
IBF - $150,000

So total cost in sanctioning fees is $1,080,000 for a $10,000,000 total purse.
This does not include the belt charge, nor does it include Sanctioning body specific regulations in regards to weigh-ins etc, nor does it include fees for officials like refs and judges.