Well, instead of making paper money, i would STRONGLY suggest you used Silver and Gold. That way its a more recognised thing. However, it SHOULD take up SOME cargo space, cause lets face it.........try hauling millions in gold around in your car. However, you could add platinum as well, to further the thing.
OR
Interesting idea for you:
How about land deeds documents?? Sink your money into realestate? You really could haul around the deeds without any space taking up, and you could have a few up to 30 odd different types, all for different values and more stuff etc. That way it would help out those that might not appreciate carrying too much stuff. You could make the deeds start at like 100k and go up to 100mil or more etc. Maybe even have deeds at 500mil or something for a city? That way you could semi have roleplay. If you had a mod with this of course, then they could trade in the deeds to own something - like a station
Of course, that takes you into the "stratoshpere" in modding, BUT - thinking more...
You could buy deeds to stations. Maybe the stations could pay out profits, or maybe not....Imagine, your station is making money for you etc, but then lots of players are hanging round, and so it runs out something for you to trade...ie mini crisis. Maybe it then costs the player to keep the station (of course, out of the way stations are cheap, things like Freeport 9 are expensive (popular) as are major ones in NY and stuff. Now they might need several large deeds for each station, which could push the prices into Multi billions...........however, realistically, you would attract massive cheaters as well. Just a thought for ya (they could also buy the deeds from teh admins to ensure no cheating......)