Vladamir,
My advice is to get the reason for the extra charges in writing from the solicitor, then let him know that you will pay the full amount (which I don't think you can or should avoid doing anyway) but only within a reasonable timeframe that suits your financial position (ie 8 to 10 weeks).
If you wish you can say that the reason you don't have the money now is that the solicitor gave you an incorrect quote in the beginning and you budgeted accordingly. The guy made a mistake, which can happen to all of us, but he can't be expecting his mistakes to happen without consequences