STOWiki has a nice article on Commodities. There's a "Strategy" section at the bottom of the page that lists the best places to buy for both Federation and KDF.

As far as the cost, I find that when I do the Tau Dewa Sector patrol daily mission, I usually pick up enough loot to sell to cover the costs of buying commodities for both Romulan and Omega Force reputation assigments. I've been mostly doing this with my main Federation character, but I would think the same would apply to KDF.