Efficiency, my son. Efficiency is what made the Roman Empire great. Build what needs to be built when it needs to be built. Don't put the cart before the horse.

Summary:

Once you have at least one crop field raised to level 5, you will have the ability to build a Grain Mill in your village. The Grain Mill will add 5% per level to your overall crop production. When is the best time to build the Grain Mill however?

Some Travian players use a system of how much it costs to build a structure vs. how long it will take to pay you back. In the long run, we believe this is meaningless. The early part of the game is an important time to maximize your resource allocations, and even a small edge can lead to you getting the jump on your neighbor, allowing you to destroy them before they can do the same to you.

Therefore, the question to be asked is "Which costs less at this moment to build per extra crop produced; another crop field level, or a Grain Mill?" Keep in mind that since gold may be used to rearrange your resources, a skilled player will consider all resources to be the same when making such calculations. If you have 40,000 crop in your village, merely spending 3 gold with the NPC Merchant will allow you to turn it into whatever you need it to be.

Assumption: You have the 25% crop bonus turned on using Travian Plus.

Case #1: All farms currently at Level 5.

Cost to build a Level 6 farm = 3250 resources which produces an extra 21.25 crops/hour for 153 resources each
Cost to build Level 1 Flour Mill = 2560 resources which produces an extra 12.375 crops/hour for 207 resources each

Case #2: All farms currently at Level 6

Cost to build a Level 7 farm = 5425 resources which produces an extra 25 crops/hour for 217 resources per crop
Cost to build Level 1 Flour Mill = 2560 resources which produces an extra 18.75 crops/hour for 136.5 resources per crop

Clearly, it's better to wait until you're about to start upgrading to crop 7 before you build your level 1 Flour Mill.