You can ask them for a quote. Outline exactly what you want and how many people it needs to be sent to and they will tell you price. It does vary wildy, they charge by the hour (more experienced they are, the more they cost) and also charge for receiving letters. Pretty sure it won't be cheap either way.
As part of mother's divorce it was about £35 a letter, but obviously the solicitor was getting paid a lot elsewhere in the case and the letters were quite standard.
The College of Law offers some free services, it's used as a way of training. They might be able togive you some free advice
http://www.college-of...tudents/pro-bono.html