He'd cost twice as much, yeah, but he'd offer us five times the quality. IMO Matuidi is nothing but a decent back-up.
I wouldn't mind seeing (in-form) Marchisio next to Pjanic, not at all, but I'd like to have somebody as a rotation while not rolling my eyes when it comes on the pitch. Also, Marchisio has been out for a long time and even when he got his chance last season he was mostly just so-so. I think it's a big risk to rely on him to perform like he used to as a DM/regista.