SEM Rush or Google keyword planner are great tools to find the high and low-range bids for your keywords. As far as formulas go, I would take the high and low ranges for all of a campaign’s keywords and create an average CPC range (the average of all low range estimates through the average of all high range estimates), then use the median CPC (split it down the middle) for your starting point. Make sure this target CPC isn’t more than your daily budget. If you’re dealing with low budgets and highly competitive keywords with CPCs around $50/click, you won’t see much volume at all. Besides that, you can also find your ideal max CPC by figuring out your click or conversion per day goals. If you need X amount of clicks per day to see a conversion, take your budget and divide it by that many clicks to get your max CPC.