Spending 500 points (max) in Heist minigame
Took me 9 hours
From rswiki talk page:
2,513 xp/reward point at 91 Hunter/Thieving (140k exp/hr)
2,356 xp/reward point at 88 Hunter/Thieving
2,303 xp/reward point at 87 Hunter/Thieving
1,770 xp/reward point at 77 Hunter/Thieving
534 xp/reward point at 40 Hunter/Thieving
449 xp/reward point at 36 Hunter/Thieving