Well make sure there is a Villiage nearby wich produces the Materials you use. For Example: if you have a Smithy, make sure there is a Villiage with Iron nearby. This makes sure taht there is always enough and not so expensive Iron around.
Basicly,= if there is enough Raw materials = the Materials will be cheap = If the Materials are Cheap = the Workshop is proftiable.
To support this, i only Raid villiages in War with produce the goods my Workshops needs. Then i sell them at the City, wich leads to an overflow of Raw Materials wich will make the Prize dropp for my Workshop.