Like I said before...if you're talking about the LARGE hole 340 driver and the Large hole 340 passenger manifolds then the only pairs you'll find for under $300 will be from a close friend or from someone who doesn't know what they have. If you're talking about the smaller 340 manifolds (1 7/8" outlet) then yes, you can find them in sets for under $300.
You still haven't stated which 340 manifolds you planned to cast?? Not knowing which manifolds you plan to cast makes the discussion very difficult!!