People don't really honor the function of currency for value. i.e. Alch + Scour function same as a Chaos however people value 3 Alch per Chaos.
That doesn't really answer your question, but basically early on things like fusings and alchs are good, later on not so much.
Early on exalted orbs sell for cheap because people don't have currency to slam things with exalted orbs. Later on, like now, exalted prices go up.
I always stock up on Exalted early, it's like investing in gold :)