Near as I can tell, the higher the number the better the price for said commodity. So, say town 1 has wine 1000 and town 2 is wine 1200. Town 2 will give a better price for wine. However, that is assuming all things are equal. That is to say that if town 2's villages are constantly getting plundered while town 1 is relatively isolated and peaceful you might be better of building a winery in town 1.