I also fit DCC chips into a wide range of locomotives. My standard fee is £50 plus the cost of the chip and any speaker. I can also fit smoke and other features at additional costs. I spend time ensuring that the exhaust beat on steam locomotives matches the rotation of the wheels as closely as possible. I try to use high quality speakers on all of my sound installations.
Some example prices are:
To supply and fit into a locomotive that requires no modification a Loksound V4 chip including speaker £150
To supply and fit into a Heljan locomotive a Loksound V4 XL chip including the supply of a high base QSI speaker and enclosure £200