Then why would you ever take the 300 wood option? You end up paying 50% more than you would if you just bought the extra 100 stone. Only reason I could see for it was if you were drowning in wood and had no gold. Seems unlikely for most realms.
It depends on what you have available. If you have surplus wood, but are poor in gold, then that would be the best option. Likewise, if you have tons of gold, but few resources, then paying everything in gold might be your best option.
Finally trading for the resources is guaranteed to be the cheapest option - but you may or not be able to find the deals you require.
