I've been using the Reefcon LT version to keep logs of my buildup and my expenses so far. I like it so far and the LT version is free, you can try it and if you like that portion and need the expanded options buy the pro version. If it proves helpful once my tank is up I'll buy the pro version to support the guys.
WARNING!!! actually keeping track of how much you spend may cause nausea, seizures, fainting, or severe chest pains.