RadMaskedNumericInput is designed to work with numbers and its Value property is of type nullable double. However, for your scenario you are working with decimal and we recommend using the RadMaskedCurrencyInput instead.
You could then either use the FormatString="P2" property or customize the currency symbol for the mask as shown below:
CultureInfo culture = new CultureInfo("en-US");
culture.NumberFormat.CurrencySymbol = "%";
culture.NumberFormat.CurrencyPositivePattern = 1;
this.uxPercentToPay.Culture = culture;