You might want to consider consulting an economist. At this point there is no player-player trading, so most of the issues seen with in-game currency shouldn't be a problem. Actually, there is really only one problem I've seen with in-game currency: runaway inflation. My guess, though, would be that you could adjust the exchange rate depending on how many people are buying cubes (ie, how many $ each cube is "worth" determined by the market). You could have it fluctuate down if demand is too low (I guess that would mean more Q$ per cube) and back up if demand spikes. I think you'd see inflation reflected as lots of purchasing of cubes
One type of inflation you also should be aware of would be the rate at which bases generate Q$, which would also be a way you could tighten the M1 money supply (you have no M2), if harvesting bases dropped in rate (ie, if it becomes less useful to players to harvest bases) then you'd say the currency was losing value.
I am NOT an economist. But I find in-game currency fascinating. I also find the pure information trades (please install App X and we will grant you N cubes) to be a genuine information age market. There is profit but neither your action (granting of Qubes) or mine (installing an app) involved any outlay of money.
I am however an internet application security engineer. If you ever want to chat I have some suggestions on how you might sniff out multiscopers.