CPC for low profits. How to determine?

Question

contextual advertising

What is the best way to determine the maximum CPC you can afford? Are there any clever formulas?
Provided that the net profit from the service performed is 200 rubles, which is 12% of the total price of the service, and the conversion is terribly low (tourism business) - 2-3%.
And what is the best thing to do if the figures given by the budget forecast generally border on profitability? Don't advertise? :(

Answer 1

did you play thimbles? adwords is the same

Answer 2

look for other advertising sources