If contracts were issued then in fairness the solicitor has probably done a good share of the overall work involved.
However I don't think that charging 50% of the fee is standard practice, albeit that they almost undoubtedly are entitled to do so.
I would suggest that you talk to them directly- if you tell them you will go with them on a future sale they might reduce the fee.
In the alternative ( since they are entitled to fees for work done) you could ask them in the event of a future sale, since they have already done half the work, will they charge half the fee on the future sale.
Talking to them directly, politely and asking for a discount, or the above is the best way to go.
No doubt they have done work, and deserve payment , my query is the amount, this is over 60% of my expectation of successful sale, and was not agreed, or signed off beforehand.
Auctioneer says he has never heard of these charges, hence my question.