There is no question ... when Oksana Grigorieva signed an agreement with Mel Gibson last May ... she got millions out of him and only gave one thing in return -- a promise to destroy "evidence," including the secret tapes.

The document obtained by TMZ -- and now at the heart of the L.A. County Sheriff's Department investigation into Mel's extortion claims -- was signed by both Mel and Oksana on May 16, after a 2-day mediation.

According to the document, Mel agrees to pay Oksana and Lucia approximately $15 million. In return Oksana agrees to "turn over all 'evidence', and represent and warrant that all evidence has been turned over and all evidence will be destroyed in a manner agreed upon by both parties."

Now get this ... "evidence" is defined in a related document as "recordings, audio tapes, emails, text messages, videotapes and photographs ... of Gibson."

A source involved in the mediation tells TMZ there's no question, "It was all about the audiotapes and everyone in the room knew it."

The signed document gives Oksana, among other things:

-- $1 million when Oksana turns over of all of the "evidence"
-- $10,000 per month for 18 additional years payable beginning on January 1, 2011.
-- A $1 million life insurance policy on Mel
-- The rights to their Getty picture deal
-- All profits from Oksana's album, totaling $17,112

There are strict confidentiality provisions attached "with financial penalties for non-compliance ... "

The document also talks about a "Joint, mutually agreeable press release about the reasons for the separation ..."

When it came to signing the formal, longer version laying out specifics, Oksana refused, now claiming she had no idea she was giving Mel unsupervised contact with Lucia. Yet in the document she signed, Mel got "50/50 physical custody" with no restrictions or conditions.

The document provides handsomely for Lucia, including:
-- $50,000 a year for the next 20 years
-- $10,000 a year for the next 20 years for college expenses
-- $4,000 a month for additional child support for 18 years
-- $4.3 million minimum when Mel dies.

Oksana's lawyer, Daniel Horowitz, tells TMZ, "When a person born in a foreign country comes to America, they look at the American justice system as the best in the world. You have a retired judge as a mediator and some of the best lawyers in L.A. in the mediation and you trust that what they are doing is lawful and proper."

Horowitz adds, "This truly underscores how victimized she was. Mel Gibson deserves better as well."

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