Nowadays the Frostweave Bag (which I assume is the one you are referring to) probably is the cheapest way to level tailoring to 450, even though it goes green at 440. What with the disenchanting option added to loot rolls infinite dust is now more plentiful and cheaper on most servers. Frostweave is always easy to farm and is cheap too on the majority of realms' auction houses.
FYI, if you are looking to farm eternals because Moonshroud, Ebonweave, and Spellweave are no longer on cooldowns or have location requirements, you don't have to go to WG. Do a search and you can find places around Northrend to farm the necessary mats that might be less crowded.
Alternatively, you could find other ways to spend the time farming something else (or doing dailies) that net you the gold to purchase the eternals on the AH. Don't forget that now you can buy eternals from Frozo
with frozen orbs.