Generally, it's not acceptable (although I can imagine edge cases where it might be okay - e.g. somebody giving an OP plenty of one-on-one tech support in comments, but after an hour's time giving up and saying "sorry, this is more time than I can give away for free".)
Either way, it's definitely not acceptable as a part of an answer, and as awoodland mentions above, it may even be eligible for a spam flag. Downvote and comment as you please.
If somebody wants to solicit small freelance jobs through Stack Overflow, they should post a notice on their profile that they can be hired, and then be generally helpful. It works! I've been contacted this way numerous times already - even though I show no indication in my profile that I am interested in such offers, and I usually have to decline.