AFAIK the 50% is pretty standard but "mid-high level galleries" both sell a lot of work for more then 5-10K, and sell a lot of work outside the exhibitions.
I know a couple moderately successful painters and they are selling bigger pieces for well north of 10K and have a lot of collector interest, i.e. it's a conversation between the galleries and the artist about what gets reserved for a show and what might go to a collector as soon as it's dry.
That's not to say they're getting rich, but I think they're doing better than your example suggests.
[Edit] Also in case it's not obvious to folks w/o art-world experience, the gallery usually does a LOT for their 50%. Basically they take over the "business development" work -- all the schmoozing, the selling, the legitimizing. At least the good ones do. It's not just paying for the wine and hanging the art. Many even help arrange studio space and pay a stipend (against your share of future sales) if you need that to stay focused on the work.