Nikki Sixx: My Famous Ex Wins Big ... If I Make $10 Million!!

Think Motley Crue bassist Nikki Sixx can rake in $10 MILLION this year?

His famous ex-wife sure hopes so ... 'cause if he hits the $10 million mark, she stands to pull in a ton of money in child support ... this according to legal docs obtained by TMZ.

Sixx divorced former "Baywatch" actress Donna D'Errico almost 10 years ago -- but they have a minor child together ... and Nikki's still on the hook for child support payments.

Sixx has already agreed to pay roughly $8,200 per month ... but she also gets a cut of any money he makes over $1.6 million ... with a cap if he makes $10 million or more.

If he hits the $10 million mark, Donna stands to get $252k per year ... not bad considering she claims she's only making $40k annually on her own.

Nikki just wrapped up his farewell tour with Motley Crue -- and it's a good bet he made some real money.

Fingers crossed, Donna.