In classical Latin, what is the best adverb for describing approximate numbers? If several work well, are there any differences? I mean saying things like "I have about ten euros". I would translate that as decem fere eurones habeo. My preference is fere, but there are other words in the same direction (ferme, circa, prope, quasi, iuxta, paene) and I do not know if there is a typical classical idiom.
You've already hit upon all the words I would use except ad as a preposition. For example, Cicero uses ad quadriginta natus esse as a synonym for fere. But in general, circa and circiter both work.
To my ears, paene would mean "almost, but not quite" rather than "around that neighborhood." Others are all general idioms for "approximately." I wouldn't, for example, use paene decem if you might have 11 euros. That word, with some unusual exceptions, seems more to mean "just about" rather than "about." See the L&S entry on it.