The attached images show the depreciating present value of receiving the ambush. For example, if we value an ambush at 4 mana and we will receive it in 6 turns at 5% (.05) interest, it is worth 2.98 mana this turn.
This can be also be seen as a lookup table of sorts for your more customized situation. For example, let us use 4 mana again, pretend that we get the ambushes in 2,5, and 8 turns, and this set of circumstances leads us to conclude a 10% (.1) interest rate. In this example, the ambushes are worth 3.30578+2.48368+1.30761=7.09707 mana. we can then remove our cost of 3 mana to get a profit of 4.09707 mana.