Creating Calculations from Check Boxes

I am trying to create a simple form with a calculation that tallies at the bottom from a list of check boxes but am not having any success. Can anyone give me some guidance on what function(s) I need to include? Below is a sample of what I am trying to achieve:
 
Check box 1 (if checked, add $25 to total fees due at bottom of form)
Check box 2 (if checked, add $30 to total fees due at bottom of form)
Check box 3 (if checked, add $35 to total fees due at bottom of form)
 
Numeric Field (calculated only, displays totals from check boxes above)
 
Thank you, thank you in advance!