Here's one opinion. Im not sure if it's right or wrong but could be worth a shot.
If it were me, here's what I'd do:
1) Identify what I mean by growth hacking
2) Set a goal for growth (Ie, I want 20 sales by Sept 30) or something
3) Set a small budget (5% of total maybe) and have them get 2 sales with it in 2 weeks
4) If they can't achieve a small goal with a small budget, I'd doubt their ability to achieve a big goal with a big one
That's just me.