I'm pretty sure the lines you're thinking of are from Sonnet 26 in Edmund Spenser's Amoretti.  Lines 11-12 run: "For easy things, that may be got at will, / Most sorts of men do set but little store."  The whole sonnet, in fact, is about the point you indicated above.  After describing various flowers that are sweet but have various negative characteristics, Spenser concludes the first section by stating: "So every sweet with sour is tempered still" (9).  The end of the sonnet notes that whatever pain is experienced now, future happiness will be worth it.