Lewis & Short

quant-ŏpĕrē (contr. form, for which, in recent editions, the MS. form quantō ŏpĕre is restored), adv., how greatly, how much (class.): dici non potest, quanto opere gaudeant, Cic. Att. 14, 6, 2; id. Tusc. 3, 3, 6.
After tanto opere, as: neque enim tanto opere hanc a Crasso disputationem desiderabam, quanto opere, etc., Cic. de Or. 1, 35, 164; v. quantus.