I think it is rather acceptable as I can't really think of a scenario when you are forced to pay real money for help within the SE.
The internal mechanism allows you to upvote, offer bounty, accept answer to reward an answerer - no real money is ever involved with the internal model.
If someone has helped you enough that you would like to pay them real money then it is completely your call. If they explicitly expose their identity in their "About Me" and are happy to accept real money for their contributions then feel free to make them but exclude SE from any inconvenience you may encounter.
You are not in any way encouraged or forced by SE to pay anyone real money for help received on the *.SE sites.
