I was also ok using around $5 a month with ptel and after a lot of research I've concluded that paygo options are limited, particularly on t-mobile. I am moving my t-mobile locked phone to us mobile for $9/mo to begin with: 100 minutes, 100 texts, 100 mbs. This is more than I will probably use, as well as more per month, but "unit" cost is cheaper than ptel, especially for data. US Mobile is cheaper than Ting.
Moving my wife's unlocked gsm phone to h2o (att) where she can probably get by with paying only $10 every 90 days for unit rates comparable to ptel.
One thing I don't understand about both of these services is why they can't offer recurring billing every month or 90 days, or auto top up when the balance hits zero. They have computers, don't they?
If I were forced to use a phone without sd storage I would just bite the bullet, get t-mobile's $30 plan with music freedom and just dig on google play music. But with 140gbs of music on my note 4 I can't really justify the extra money.