Depends on what stage of the game your in. Early in the game, you might get resources for 10 gold/turn, later in the game you may have to pay 300 gold/turn, or more! I would shop around and find the lowest price amongst all civs that can trade it to you.
There are other ways to get a resource, if you're lucky, without declaring war. If the resource is just inside their border, then plant a city, just outside their border, rush buy cultural improvements, and hopefully your border when it expands will take over that resource. Or your city will take over their city by culture.