To be fair, T-Mobile doesn't advertise their plans as "unlimited", they advertise them as 500mb/2.5gb/5gb at 4G speed. It's only $10/month more to bump up to 2.5GB.
And $10 more after that to go to full blown unlimited.
And if you paid the extra $20 a month, you would have the Unlimited LTE plan instead of the Unlimited 2G plan with 500mB of LTE. $70 is still reasonable in my eyes.
I have unlimited data on Virgin Mobile for $35/$45/$55 a month (based on # of minutes you want), though I had to pay full price for my phone.
I don't know which TMobile plan you're on. I have "unlimited" which throttles to 2G after 5GB, for only $30.