An ex Lib-Dem minister (anybody remember them???) had said in 2013 that the Tories wanted to set the earnings threshold for getting a spouse from outside the UK to £40,000, but that the Lib Dems forced it down to £18,600. Now that the Tories have a majority of their own and the Lib-Dems have been rewarded with a thrashing for having been in government...
http://www.telegraph.co.uk/news/politic ... rants.html
Based on past form (and not on any hearing), I would expect the earnings threshold to be indexed to the rate of inflation every year. At worse, it could be like the triple lock on pensions; earnings growth, inflation or 2.5%, whichever is highest. But this statement is pure conjecture and not a prediction.
As regards the request for the Tory party manifesto, Google is surprisingly efficient at online search and revealed the link below;
https://s3-eu-west-1.amazonaws.com/mani ... to2015.pdf
It does not mention the earnings threshold at all. There is of course mention of improving the required standard of English, but that is broadly it as regards non-EU immigration.