There's a 10 year agreement with NFL to play a minimum of 2 games a year at the stadium, with a view to the possibility of having London based NFL team play all of their games there in future.
The grass pitch sits over the artificial turf, and when there's a non-football event on then the pitch slides away underneath the stand. It's all mechanised and quite impressive. Tech that Wimbledon (Grass Court Tennis) uses on a smaller scale.
This is all just aimed at making the most use of the stadium and which would ordinarily only be used one/twice a week at most. They're just commercial revenue generators, and it's a shot at tapping into American market, which Spurs have been pushing heavily over the last few years.
The hand-egg pitch needs to be lower down, due to all the (6ft plus) subs standing on the sidelines, again hence the mechanised retractable pitch. At Wembley they just fenced off the first few rows.
Impossible to say, but estimates are around £750m to £1bn.