Little-scale's extensive work with Sega sound synthesis brings this in-depth tutorial - This tutorial is a step-by-step guide on how to make your very own 32KB Sega Master System cartridge. The cost is quite low (if you have a programmed EPROM or you are able to program your own) - I payed around AU$8 in parts for one of my carts as made in this tutorial.
These carts have been tested on a PAL SMS 1 and a PAL SMS 2.
The good news is that the cartridges use a re-programmable EPROM, provided that you have the appropriate facilities (EPROM programmer and EPROM eraser). - How To Make a 32KB Sega Master System Cartridge
http://blog.makezine.com/archive/2008/06/how_to_make_a_sega_master.html