So it made sense I thought to move to the 90GB plan (rather than the more pragmatic option of scaling back on family Internet usage and putting in an IP blocker!). But it turns out not to be the case.
60GB is $95.95 and 90GN is $139.95, a $44 extra. But 30GB of ad hoc data is 15x$2.95 or $44.25 so you save the princely sum of 25c if you prepay. And while I haven't reached 90GB in a month yet, if you don't then you lose out more.
TCL needs to really rethink their plan pricings.