Single lines on all major carriers always seem to go this way. They'd rather get you locked in multiple lines. Two lines with 10gb is $100, so you've already drastically jumped up in value there, assuming you have two lines you could use. Keep in mind music streaming doesn't count against your allotment; my wife and mother could very well do with just a basic 1gb plan at that rate (the 10gb still comes out cheaper)I'm on T-mobile (through another, much cheaper reseller) and although they seem to be better in every way than the other carriers, their plans do just seem a little high to me. A 3GB LTE plan is $60/month, whereas I have my wife on Cricket's 2.5GB plan for $35 (including taxes and fees).
Maybe their value is more apparent when you can get a bunch of lines on the same plan, but for an individual it just seems high.
Pay as you go is almost always the way with single lines IMO. Once you get multiple lines you tend to get multiple perks, and of course, better financing options on phones if you upgrade yearly.
TL;DR their single lines aren't a good value. I doubt very many people are using them.