According to Spanish football journalist Guillem Balague (the same dude who reported yesterday that Cesc Fabregas won't be leaving Barcelona this summer), Thiago Alcantara indeed has had his release clause reduced from a staggering €90 million to €18 million. This is because the midfielder has not featured for a minimum of 30 minutes in 60% of Barcelona's matches this season. If any club wants to trigger that clause, they have until August 1 to do so.
As you're probably aware, Manchester United are a club that have been linked to the talented 22-year-old -- and this interest reportedly goes all the way back to the summer of 2011. Thiago has been brought up at the famed La Masia academy and he's thought to be the eventual successor to Xavi. However, he's apparently open to the idea of moving to another club so that he can feature more regularly. United are not a club, like Barcelona, with a midfield full of world-class midfielders. Therefore, his desires for more playing time could likely be satisfied at Old Trafford.
One interesting tidbit from Balague's report is that Thiago's "preferred position is closer to goal, more like an [Andres] Iniesta." The roles of Barcelona's midfielders in their 4-3-3 system is different from United's 4-2-3-1/4-4-1-1ish system*: the 'Xavi role' is something in between a holding and a box-to-box role as he's generally the one tasked with dictating games as a somewhat deep-lying playmaker while the 'Iniesta role' is something in between a box-to-box and attacking-midfield role as he's the one tasked with being the most incisive of the midfield trio.
If Thiago were to come to United, his main role would likely be as a partner to Michael Carrick deep in midfield, but with a license to venture forward safe in the knowledge that the current United player of the year will protect the back four. Other roles that he could be considered for would be as the most advanced midfielder (the 'Iniesta role') if Moyes ever decided to deploy his new side in a 4-3-3 shape or as a No.10 when Shinji Kagawa or Wayne Rooney (if he's still around next season) are unavailable.
Assuming Balague's report is accurate**, then this seems like a no-brainer: United should trigger the release clause before August 1 and then look to convince the gifted 22-year-old that Old Trafford is the place for him. It's not often that a potentially world-class deep-lying playmaker is available for such a bargain price and at such a young age.
** Balague isn't always accurate, but he does have good knowledge of and sources at FC Barcelona.