To cite like [1, p. 101]

\cite[righttext={\nbsp p.~101}][Hankins:2014]

works, but has no comma

\cite[righttext={, p.~101}][Hankins:2014]

produces no page

\cite[righttext={,\nbsp p.~101}][Hankins:2014]

gives no page at all

\cite[righttext={\nbsp, p.~101}][Hankins:2014]

gives no page at all.

righttext seems to be reluctant over the initial comma right after the
reference number.