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If you're still in doubt with your builders opinion, get a second opinion from a local architect, builder, or engineer.
Using TJIs, and sometimes with open web floor trusses, it is possible to have longer spans at less net cost because some beams can be eliminated and labor cost can be reduced. In other situations, cost can go up significantly. It takes experience to know when and how.
The best you can do, with your limitations, is to go to the local lumber supply that sells engineered joists and show them what you are trying to do and they may be able to help you with engineering software and cost estimates. But be prepared to follow through and buy from them, they shouldn't be expected to do this work for free.