If he's on 5 grand a week, quite a bit lower than the average wage at Rangers, he'll still earn over a million pounds over four years.
To stay at Motherwell would literally be like winning the lottery, then tearing up your ticket so you can keep working at McDonalds.
On top of that he's played 14 more games and scored 3 more goals than he had this time last year at Motherwell, ie none, in front of much bigger home crowds (Rotherham 8,000 - 10,000 ave attendance) and at a level that's no worse, and arguably better than he'd be playing most weeks in the Premiership.