Barcelona and Real Madrid tend to treat La Liga players and in particular the Spanish U21 players as "theirs" and merely on extended loan to other clubs. Barcelona just haven't (hadn't?) completely accepted the idea that they might then have to pay full market value for them.
Real Madrid took Isco and Illarra a little earlier than normal and paid their valuations. Barca weren't willing to do that to get Javi Martinez, despite needing a central defender, so buying an expensive player like Herrera as a contingency or for the future certainly wasn't their style.
To be fair, looking back at the Herrera transfer thread on here (2013 and 2014) a lot of people were saying that 15-20m would be reasonable, £25m if pushed and Barcelona had the same opinion. If they're debating it again now - good, it means he's playing well. It would be more surprising if they weren't.
However, he's our player and the more games he plays, the more he looks like the player I hoped he was - a proper United player.