Find standard deviation of costs from different sources

Hi everyone,

I am trying to find the standard deviation of the cost of a certain health treatment. The cost consists of:
  • Out of pocket costs, sourced from a survey, from which I can find standard deviation.
  • Health insurance costs, for which I have a minimum/maximum, from which I can estimate standard deviation.
  • Government costs, which are fixed payments and therefore have a standard deviation of 0.

The average overall cost of treatment is the sum of the means of the above. To find the standard deviation of this overall cost, do I simply sum the standard deviations, or is there a formula for this?

Thanks for your help,