The couple split in April (10) but the break-up turned nasty when the Russian singer accused Gibson of beating her and knocking out two of her teeth during a violent argument in January (10).
Grigorieva taped phone arguments between the pair and was rumoured to have been offered the multi-million dollar sum by Gibson's legal team to settle the custody fight over their baby daughter Lucia and keep the news of the audio footage secret.
However, the tapes subsequently leaked online - and now Grigorieva has spoken out to address claims she's holding out for more cash, insisting her reasons for walking away from the deal go deeper than the balance of her bank account.
Grigorieva tells People magazine, "He basically offered to settle it (custody dispute)... (but) the $15 million was a proposal that I walked away from because he was wanting 50 per cent custody of an infant when he's unstable. I feared I would never be able to prove that the tapes are accurate and not doctored."
In the same candid interview, the singer alleges Gibson's personal troubles stem from his days of alcoholism, insisting he's in need of treatment to rid him of his "old dry drunk rage".