In fairness to all, we don't discuss pricing on the Wall. Thanks for your cooperation.
You're trying to vent an atmospherically vented appliance through a 3 ft. vent and wonder why there's no draft? Now, you're going to add a 90 ell along with a 5 footer? Why the 90?
Thos gas pressure numbers don't make sense. Please clarify are you talking about manifold pressures and when you say "3 inches", do you mean 0.03 inches w.c.? What are your inlet gas pressures?
All the makeup air in the world won't cure a short, cold stack.
With the room full of warm air, the stack effect in the room makes the room a better chimney than that crummy 3 foot pipe.
Rudy, I hate to say it but if you're calling 4" B-vent expensive, you're in way over your head.