chall,
Instead of buing a new interface cable for each new sim (BTW, $19 FMS cable only works under Windows 98/ME), you can buy a universal gameport or USB interface, which will work with (almost) all simulators. Only those sims, which intentionally disable support for all 3rd-party interfaces (like it's done in RealFlight G2-Interlink) will not work with such interfaces. Everything else, e.g. non-Interlink G2, AeroFLY and AeroFLY Pro, RC Plane Master, FMS, PRE-Flight, etc., will work fine (I tested such an interface with all aforementioned sims).

Right now a TX-to-Gameport interface can be purchased for about $30 (only $10 more that a FMS-only Win98 cable), e.g. here:
www.milehighwings.com/joystick.htm
and I expect the price of a game port cable to fall even lower as USB interfaces gain momentum.