I guess I think about this way, at least in terms of the maintenance: If I were arranging that city's citizens, would I have them work 2 food and 1 production, or 2 gold? I mean, it's not even close... I'd pick 2 food 1 production over 5 gold probably. The 120 hammer cost is a bit of an entry barrier, which is why I prefer to purchase these and build Granaries, when possible.