Generally those venues pay union minimum + housing, transportation, per diem, etc. Similar to off-Bway venues like the Public or the Broadway nonprofits. The pay isn't great. Sometimes the theatres even have policies in place where all the actors must be paid the same amount.
If commercial producers are attached, that's a different story. There could be a "holding fee" paid out by the commercial producer to help make up for a small LORT salary. Or they may owe the actors $X per week if they don't offer the actors a contract for the Broadway production. And separately, the tier of workshop/lab agreements with profit participation is a factor. (After HAMILTON moved to Broadway, those producers famously paid additional money to those who had appeared in the Public production to offset that salary, in addition to profit participation)
If you're a summer stock company or a lower-tier theatre or guest-artist-contract company, you should expect to pay through the nose for any artist who could "sell tickets" or boost your profile.
