You aren't going to get a valid answer here. Call some local contractors, let them see the job and let them give you a price. You're going to have to talk to a local contractor sooner or later, as no one here is going to do the work, so it may as well be sooner.

I have a sloped backyard in Dana point, ca.

It's about 50 feet wide by 40 ft deep

Trying to get an estimate for removing dirt and building a retaining wall .

There's quite a bit of dirt to remove. The slope isn't too crazy

The height of the walls has to be about 10 feet on side and 20 ft in the back

I have no idea and just want to get an idea . Should I expect 20k or 50k ?

Looking for block wall