Let's take a look at the choices:
AT&T: Ma Bell is the only US operator (currently) to say it will charge for mobile 5G by the gigabyte. In launching its mobile 5G service at the end of December 2018, AT&T said it plans to charge $70 a month per 15 GB of data on its 39GHz millimeter wave 3GPP-based mobile service. Customers can use the Netgear Nighthawk 5G hotspot -- due to hit the shops in the coming months with a $499 price tag -- to connect to the 5G network.
T-Mobile US: The "Un-carrier," which is yet to launch its 5G service, says it won't charge a premium for its 5G plans (compared with its 4G pricing). T-Mobile is expecting to launch 5G service in 30 cities in the second half of 2019. (See T-Mobile Won't Charge a Premium for 5G and T-Mobile Quietly Confirms 5G Network in 30 Cities.)
Verizon: The largest mobile operator in the US recently announced that its initial mobile 5G (mmWave-based) service will become available in parts of Chicago and Minneapolis from April 11. For $10 per month, Verizon customers can add a new 5G unlimited plan to their existing 4G LTE unlimited plan. This means the minimum total price of a 4G plan with a 5G premium would be $85 per month, as Verizon's 4G plan prices range from $75 to $95. Only the Motorola z3 smartphone with a Motorola 5G Mod snap-on module will be able to connect to the 28GHz 3GPP-based mobile 5G service initially. Verizon is offering the 5G Mod snap-on for $50, while the z3 phone costs $349. Verizon plans to launch its 5G service in 30 cities this year. (See Verizon Says Its Mobile 5G Service Is Really Unlimited and Verizon Announces First Mobile 5G Cities & Mobile Moto Module.)
Sprint: The only major US mobile carrier yet to reveal anything about its 5G pricing plans. The proposed merger with T-Mobile might be the reason for its silence, but it hasn't held back T-Mobile from sharing its strategy.
Early days: Expect 5G contract pricing to evolve as services become more widely available. It's likely that different pricing models will emerge as new applications -- such as high-resolution, real-time gaming and augmented/virtual reality services -- become mainstream. (See 5G in the USA: Going Mobile.)
— Dan Jones, Mobile Editor, Light Reading