Is that what your conveyancing solicitors have quoted you for sending an appropriate letter? Seems expensive for one letter even on a charge out rate basis! Is it for just one initial letter (bear in mind the lenders may then want to be kept updated and informed as to progress by them) any information needed from the lenders throughout?
It doesn't surprise me some firms would but there are many others that wouldn't and do it as part of the service or add on an appropriate amount to the final bill at the completion of the matter as the bill is usually deducted from the sales proceeds (will there be enough?).
You could try explaining the issue to the lender and asking if other information would suffice such as confirmation from Estate Agents or a general letter of the current position from your solicitors to you by way of update say.
It is common to demand it from the lawyers themselves though as to progress. I'd definitely query the fee or time of payment with your lawyers!