If you want the user to enter an amount within a range, for example, amounts between $300 and $1,000, and then pay for it, you can use the custom validation field.
For example, the following RegEx validation rule helps you accept numbers with up to 2 decimal points.
/^(?:[0-9]{1,5}|[0-9]{0,5}\.[0-9]{1,2})$/
Learn more about how you can set customized validation on form fields (RegEx) to make more validations and collect more unified data on your forms.
You can test your RegEx patterns and learn more about validation rules on this website.