Xfinity's pricing structure is a bit different from most carriers and is best utilized by people who want to maintain a lot of lines but don't use much data. If you sign up for the minimum "by the gig" plan, you only pay $15/month for the first gigabyte of data you use on any of the 1-5 lines you can maintain on your account. You do not pay any extra for voice or text. If you have 1 line and you use 999 mbs, the base price is the same if you have 5 lines and use the same amount of data. The only difference is that you also pay about $2/month on taxes and fees.
If you want your lines to have more data, you can choose $30/month for up to 3GB or $60/month for up to 10GB.
For early adopters like me, Xfinity used to not charge anything except taxes if your lines used less than 100mb in a given month. Now, the minimum applies whether or not you use any data. I am grandfathered, however, into paying only $12/month, the only drawback being that my lines don't have access to 5G.
Finally, you have to stay with Xfinity home internet to maintain this pricing. If you terminate your home internet but keep Xfinity Mobile, you pay an extra $20/line.
At any given time, I maintain 3-5 lines that don't use much data, so I pay about $20/month. That will probably change to 4-7 lines going forward ow that I know I am actually allowed up to 7 lines. I usually make up for whatever I have to pay (and more) by churning lines to take advantage of their $50 BYOD promotions and by taking advantage of their periodic iPhone upgrade deals (I acquire new phones, qualify for $200-$300 gift cards, unlock the phones, and re-sell them for profit).
It appears that Xfinity's 60-day device unlock policy may not be a hard and fast rule. I bought an iPhone SE on Sep. 19, paying the full price of $399 (plus tax). I received the phone on the 23rd and activated on the 24th. With the upgrade an existing line deal, I will qualify for a $300 GC sent via snail mail. Earlier today, I called a live agent to request a device unlock. It was granted and I've verified that the SE is SIM-free in practice.
A few others on the SD forum have also experienced the same thing. If you pay full price and are a long-time customer, Xfinity seems to be lenient on unlocking.