Celebrity Realtor Josh Altman Says Ben & J Lo Will Take Loss in House Sale

Josh Altman says Ben Affleck and Jennifer Lopez's mega mansion is appropriately listed at $65 million ... 'cause they're probably gonna take a hit, which is factored into the price.

We caught up with the celeb real estate broker -- who's repped a ton of stars in his career -- in L.A. Wednesday, where he told us why he thought the high price tag for their crib, which they're trying to sell quietly, is actually perfect ... getting into the nitty gritty with us.

Check it out ... JA says he's very familiar with their property, and thinks they got it for a steal at $60 mil when they bought it last year -- but because of the new mansion tax in Cali, not to mention commission they'll have to pay ... they're already in the hole at about $6 mil.

Because of that, he believes Ben and Jen's managers have told them to try and sell this palace at $65 million -- which, if they can get it, will allow them to basically break even.

While Josh is optimistic they can get that ... other sources have told us they stand to lose a lot more -- at this point it remains to be seen who'll bite ... if anybody.

Josh had some more insights for us in this chat -- getting into the fact that stars regularly buy and sell houses in short windows for a number of reasons ... although, in this case, it seems obvious that BA and JL are trying to unload because their marriage is on the rocks.

He also weighed in on what type of buyer might be lining up to snap this place up from them -- spoiler alert ... it'll be someone who's a big hitter, and who's flush with cash.