It depends on how "realistic" you want to be.
I really cannot recommend strongly enough, even though they are far from free, buying Expeditious Retreats "A Magical Mediieval Society: Western Europe" and "Silk Road".
They will give you excellent economic info, based off of the DMG (even the 3.5 DMG, now) and built upon from an impressive list of references used for as much "realism" as possible.
It can even get you realistic numbers, using worksheets, for Manors, villages, towns, and cities.
So whenever your game starts getting into settling and building lands, these books are great.