I'm building a 4300 sq. ft. home in South Texas and my builder wants $900 to change the ceilings from R38 to R44 and $800 to change the walls from R13 to R15. Is this worth it because the entire roofs have Radiant Barriers already installed.

Not enough information.

Foam? Batts? Blown fiber?

It makes a difference!

High density R15 batts are more about the reduced air infiltration than actual R-value. If the framing is caulked to the sheathing inside every stud bay and all penetrations of the framing & sheathing are foam sealed the improvement in "whole-wall R". The quality of the installation matters too- perfectly installed R13s will beat poorly installed R15s.

Dana 1 Thank you for your impute it was most helpful. Jimmy