For an estimate:
At level 99 Ranged you'd gain an average of 280 exp / chin.
At 85 Ranged you'd gain 280(85/99)^1.1 = ~235 exp / chin
(280+235)/2= ~258
(13,034,431-3,258,594)/258=37890
37890 * 807 = 30.5M
As for prayer potions, I think 500 potions should be sufficient, this however depends alot on your gear. 500 should be sufficient if you are using good gear.
500 * 2500 = 1.25
30.5+1.25= 31.75M
Obviously this will not be exact, however this should give you quite an accurate idea of how much you will be spending.
Cheers, Creative.