T-Mobile prepaid plans are great in that regard. I only pay the New York sales tax on top of the quoted price (8.875% or something). My parents were on AT&T and they were paying close to $15 in mysterious fees + taxes per line, for dumbphone lines without data plans. I convinced them to move to T-Mobile prepaid, seems to work well so far.

Normal sales tax only. No other taxes/fees - they're all included in the basic rate. This is generally true of prepaid plans across all US carriers.