There are some money system implementations that work across grid-boundaries (eg. OMC or paypal) However those usually require specially adapted sales scripts.
At the same time when implementing a money system for your grid, you need to think about why you want iz? Do you want members to be able to trade only inside your grid (meaning the currency cannot be exchanged into rw money) or if you want a currency that can be converted to rw money (which leads to lots and lots of interesting legal questions and considerations ....)
IMHO having an inworld currency with no rw exchange should be enough for most grids, and that is why I am waiting (more or less

) patiently for photo's money modul
Regards,
SilverDay