Yes, you can turn off mobile data and use WiFi & Xfinity hotspots instead. That's what I do to avoid the $12/GB charge.
I registered my 1st line in June and so far, I only paid $2.19 a month for 911, State sales tax and city sales tax for it. When you receive your new phone from XM, do not power it up yet. Remove the XM SIM from the phone, power it on, disable Mobile Data, set up WiFi, run firmware and software updates first. These updates might swallow up the free 100MB data allowance so you'd better do them on WiFi. Then turn off the phone to insert the SIM back and start the activation. I would keep Mobile Data disabled then run the Internet connection via my WiFi and the many xfinitywifi hotspots around the city. Remeber Xfinity Mobile calls, SMS and MMS don't need mobile data to operate. In fact, my Xfinity Internet was cut off for about a week during Hurricane Harvey and I disabled Mobile Data during that time. I was able to make/receive calls and texts, even MMS picture texts, to/from friends and family. So just disable mobile data to avoid the $12/GB charges.
I activated my phone the regular way, unlike SBM's process, and it did eat 8MB to get activated. Then I shut off mobile data and connected via Wifi, downloaded apps and worked on customizing the layout, sync'd my google email and calendar etc all via wifi and it worked great. Especially if you have an Internet modem that broadcast the 5mhz high speed connection. Really fast. But be aware that even with the mobile data off you will find over time 3 or 4MB's of data being added to your usage. It is all for checking for updates to the operating system. You can watch it grow over the month in the mobile data section. For the last 2 months I have used < 10MB's mobile data and >10Gb's of wifi data and only charged the rental fee and the state taxes also $2.19.