PPC Bid Rate

Please any buddy can tell me that how we decide the bid rate for the PPC campaign what is the formula of finding the maximum bid rate if we don't know the profit per sale.

i guess it depends on the type of business you're selling to. but i say its not worth the money spent. you have to make atleast 30% to cover costs and adv. in my line of business.... then making a little for your self afterwards.

there is no fornula for max bid rate if you donno the profits

it doesnt make sense to do that at all....