This assumes that the provider pays the same amount for the phone as you would if you purchased the phone then looked for a network.
But because the provider can purchase in bulk and will have a break even point, so what is the true cost of the phone from the provider?.
For example provider X sees a phone retailing for £400, but goes to the supplier and ask for a discount for bulk. and provides the you with the phone at £25 pm. over 24 months
You do the calculations mmm. phone cost £16.67 pm and looks at the pay as you go costs, say average 100 minutes at £0.4per minute per month (£40 in call charges) then say 200 text at £0.12 pm (£24 in texting) then 500mb of dater at £1/mb (£5).
So for £25 pm you get a phone for £16.67 + £69 in calls etc.= £75.67 pm.

But say I purchase the phone completely then go for a rolling unlimited contract of £15pm
so £16.67 + £15 = £31.67

So with these figures how much does the provider actually pay for the contract phone?

I cannot find out this figure, because it is a trade secret.