It's impossible to say what would be a good fee and you will find it hard to pin any solicitor down to a quote until they know all the facts, you will need to meet with the solicitor, perhaps even 2 or 3 different ones. If you have employed an auctioneer who specializes in this sort of thing they will know the solicitors who sell dev land regularly. There are a lot of ways to save money when selling dev land and a solicitor who knows what they are doing can often earn their fee by saving you money in other ways. Talk to a couple about your particular situation, see what they are offering, then decide what is good value.