Drake Debonn:
I don't even know if I understand the day units.
There should be a thread devoted to explaining them over in RoA OOC Forum. If not, they should be explained somewhere or other in the player's information in the OOC Forum.
In a nutshell, you use them to represent things like training, research, rest and relaxation, item creation, and so on. Some DMs use day units to reflect travel to far and wide places (in an adventure or especially when a player and his/her PC moves from one region to another in mid-adventure).
If your PC is attaining a new class, creating some item, learning some feat that is something of a stretch (a PC in a forest area learning feats having to do with the sea, for instance), etc., see ow many day units your DM will ask you to burn or spend for doing those things.
Stored day units never go away, so if you are planning on something big, save them up.
Costs are usually left up to the DM. If there is a doubt, we DMs talk about it in the DM's Lounge and come up with a reasonable amount to "charge".