How to measure a dough humidity? Different flours have different levels of humidity, or they absorb different amounts of water during mixing. It would be useful to be able to measure the dough humidity to judge the ratio of ingredients and avoid a failed batch.

This is especially useful when variations on the choice of flours are done, so that the "tested" formula does not apply anymore.

Are there cheap tools to do it?
You don't need anything special, just a scale. Put a trivet, then a pan on a scale and tare it. Next, pour in a certain amount of flour, say 100 grams. After that heat the pan and therefore the flour up on the stove, which will evaporate the water, then re-weigh (that's why you have a trivet: you don't melt your scale). The difference between measurements will tell you the humidity of your flour. 

I personally don't think there's much to be gained knowing this. Recipes are written accounting for water content in flour, they don't expect flour that's completely dry. I've baked a lot and I've never had a situation where the humidity of the flour has made a noticeable difference, especially when considering other environmental variations.