"Assuming that Castania (or Robert Greene) did not make up the words attributed to Propertius" ... that assumption is probably not warranted :-) Just as Greene invented biological phenomena out of whole cloth, he did much the same thing with classical quotations.
In Carmine G. di Blase's chapter "Euphuism and the Uses of Falseness" in Trompe(-)l'oeil: imitation and falsification by Philippe Romanski and Aïssatou Sy-Wonyu is an analysis of Greene and other writers of the period by someone who seems to know what they're talking about:
Sidney's fear, according to John Hunter, was that Euphuism, with its endless accumulations of examples, had become "a threat to the value of an educated memory" [J. Hunter 51]. [...] Sidney's Defense was written in 1583, when Greene was making his first appearances in print. Greene, I believe, was at that time already struggling with the very same problem. For Greene, this struggle manifested itself in quiet, but deliberate and persistent, falsifications of the knowledge that Euphuism was expected to impart. His falsifications may be found not only in his dedicatory letters but throughout his Euphuistic works. He was careful enough with the more common classical allusions. What he says about Medusa, for example, or Sisyphus, or Pyramus and Thisbe, is generally correct. But when he needs an authoritative judgement on any given subject, he is liable to invent one. It is unlikely, for example, that Propertius ever said "that to love, howsoever it be, is to lose, and to fancy, how chary so ever thy choice be, is to have an ill chance" , and how can one know if Plato ever really said "things lightly granted (though never so costly) are smally accounted of" ? According to James Applegate, who has attempted to annotate the classical allusions in all of Greene's works, every opinion on love that Greene attributes to Propertius is "unremarkable" [Applegate 281]; and the same is true of Greene's allusions to Plato, "many of them being so inane as to make it almost certain that Greene is merely attaching Plato's authority to a remark which he finds it convenient to make" [Applegate 272]. And such is the case with greene's treatment of nearly every classical authority.
Don't waste your time looking for this quote in Propertius: Greene was most likely making it up.