Yes, it simply sounds like you need a more robust budget. In my budget in Excel I have a line for each reward. It's actual costs, it's postage and packaging costs, and my profit margin. I also estimate how many people I think will pledge at that level. I estimate the spending habits of my usual customers - how many might purchase at $10 or $20 and how many might go for the higher level. To be fair I had my first successful project to use for estimates.
However, my higher level rewards actualy have a very low additional cost. For example, my $500 reward actually only has about a $5 additional cost to me (other than time) over the $100 level. And the $100 level has about an extra $5 cost over the $25 level. (though I work in GBP rather than dollars, of course) So I'd rather more high end backers than low end backers, but my budget is based on more low end to be safe.
I do have formulas but it is actually a couple of Excel sheets of costings, pricing, profit margins, buyer number estimates, my goal income for my time, and adding the thing I am actually trying to raise additional funds for beyond the reward products. (in my case, funds for my art gallery) So not one simple answer or formula really - it's more like an overall business plan with budget and sales projections. You can find sample things like that online for new small businesses if it would help.
Tina.