I think once you get into (and I know this is somewhat an arbitrary figure) 60k+ per week, you're starting to say 'we believe in you as THE left back and perhaps more than the actual guy we bought to play there for 25 million in Tierney'.
We shouldn't be going overboard, it's not even been a full half season of convincing he's done, he doesn't actually even want to play left back and we have 2 other competent left backs! He hasn't come anywhere near to proving his quality beyond doubt at left wing too, which is ultimately where he wants to play. He will need to outdo Martinelli in the short term for that.
I would say up to 40k...perhaps a little bit beyond. I worry with his comments of 'I leave that down to my parents and agent' that we will be subjected to the greed of his parents and/or agent though. You're 18 now kiddo, that's a gown arse man in the eyes of the law....it's down to you.
But hey, I get why they peddle that line...