Validate a currency string
Since


  EnteredCost and EnteredPaymentReceived are input parameters from MaskedTextBox


and a MaskedTextBox


  Uses a mask to distinguish between proper and improper user input.


then the validation is not necessary because all it does is to check whether the strings contain numeric data and a dot . as a separator. The MaskedTextBoxs alredy do that so why are you validating their input again?