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J Lo and Ben In Escrow for Insanely Expensive Estate ... Families are About to Blend!!!

Jennifer Lopez and Ben Affleck are taking the next step in their relationship, and it's a big one ... they're plunking down more than $50 MILLION on a spectacular estate ... real estate sources tell TMZ.

We've learned they're buying a 20,000 square foot estate in the ritzy Bel-Air area of Los Angeles. The home has 10 bedrooms and 17 bathrooms!!! It has multiple kitchens, a theatre, gym, his and hers bath, and on and on. Oh, of course, a pool.

The house is super private ... tucked away in a forest of trees. It has an incredible city view.

We're told they sealed the deal in the mid-$50 million range.

Our sources say it's not a done deal because there are still contingencies that must be satisfied.

It's a big move, and we're told the entire brood from both sides will be living there. It certainly seems like a precursor to marriage.

The home is listed for $65 million, so the seller went down in price. And, speaking of the seller, his name is Todd Lemkin, a Texas billionaire.

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