You would simply create an average CPC...
Go to google keyword planner,select all the keywords you would like to bid on.
Then export,go to excel and highlight the cell 'suggested bid' then in the bottom right it will show you an average cpc.
With the average cpc you can then work it around two ways:
1.) Based on the amount of clicks you want.
2.) Based on the amount of budget you want to spend.
For a forecast I would generally create 3 different proposals at 3 different amounts of clicks or 3 different budgets.
Let me know if you need any more help.