Baseline should be hinted at by NPC pricing, is how I'd go.
NPCs have a pretty "bold" sell:buy rate, in order to leave plenty of room for player to price in-between.
For example, if an NPC will sell to you copper ingots for 1,000 gil, no player should ever price it above 1,000 gil. Components to make it should also fall in line, and be a fraction of 1,000 gil.
Also, if NPC buys the ingots for 200 gil, then no player should really sell theirs below that, as they can get better and faster money selling directly.
If the NPC-selling price is unknown, you can always try to guess it by multiplying their buy price.
Obviously, as time passes, player supply and demand is ultimately what determine the real value of things, but using the NPCs as a starting point seems like it will be the best bet at first.
The problem arises when NPC sell you finished products below the price of the components. You have to sell your finished product below NPC price to get any sales, yet you are losing money if you sell that low.
(The argument is often that once I turn copper into gloves, all you can do is wear the gloves. Should I sell you the ore, you're free to make it into many other products, therefore the value of the components is superior to the value of the finished product.)
FFXIV: Cloe Delisle
Scholar, officer of the SWAGGER Free company, Sargatanas server.