'The' or 'a' is normally implicit, if not actually stated, when the phrase is used.
A. Have you bedded the new barmaid?
B. Chance would be a fine thing!
Surely B is saying that he would very much appreciate the opportunity, but - perhaps because he is a drunk, is getting on in years, is ugly or whatever else might preclude her looking kindly upon him - he realises that there isn't much likelihood that she ever will.
The OED isn't much help on this one. Partridge suggests the phrase has virtually become a proverb. The earliest example he could find is in the play, Hindle Wakes, dated 1912, though he also suspects it nay date back to Restoration times.