Sources connected with Oksana tell TMZ ... she has five reasons for needing 20k every 30 days:
-- Lucia is Mel's daughter, so she needs round-the-clock security
-- Oksana is in debt for more than $100,000 -- for legal fees, PR costs, etc -- and she needs to dig out of the hole
-- Oksana says Mel promised to take care of her and his daughter
-- They got accustomed to the Mel Gibson lifestyle
-- The baby costs a lot, with medical costs, food, clothing, gas for the car, etc.
So, we ask ...