There are plenty of quality midfielder we could get for £30m or less, or even less then 20 in some cases, all who would improve our first team or able replacements for Carrick.
Koke and gundogan would probably come if we bid £30m and good enough wages (although it's far from guaranteed), then there are others like cabaye or Schneiderlin in the premier league who would improve us and replace Carrick. I doubt they would be very expensive either. In the serie a there's marchisio at juve, pjanic at Roma, and probably a few others who I can't remember off the top of my head who would be attainable and would instantly improve us.
Matuidi's contract is expiring in the summer and it would be fantastic if we at least tried to get on a bosman in the summer, but we could probably bid less then 10m and get him in January if we wanted and he wanted to come.
Then in Germany there are loads of quality midfielder, as well as Spain, like the benders, Herrera, etc.
Basically, there are loads of quality midfielders out there who we could get, if we bid the right amount. It doesn't have to be anything ridiculous either, but if we're thinking of getting them for bargain prices then we can rule out ever signing a quality midfielder basically. We just need to be decisive and show we actually want to sign them early on, rather then feck about all window and panic at the end.