Look this is just BASIC negotiation skills. You dont just give away 20% because you THINK thats what they want, you ask them after you have burst their bubble. If they say 20% you can suck in air and say OK if you go on contract or pre pay.
What if they ask for 10% more off whatever figure you give them? Would you do it for 30% off?
If you asked them and they said well we were hoping for 5% off, would you say no, no, no I will give you 20% off because I like you.
But hey its your business, do what you want.