How to get rid of special char? Help please!

I'm having a hard time creating a form field that only accepting number.

The ideal way would be using ajax or javascript but I don't know how so I tried to use form submit instead.

There are 2 text fields & one drop down where user can enter dollar amount, then select a fee from a drop down.

The 3rd textbox will then display the total amount from 1st field +  2nd field, real simple.

1. Enter amount, for example, user enter $10 on the 1st. field then

2 Select fee range : $1 to $200     : $20 <<<< for example user select this value

                             $201 to $500  : $30

                             etc

then,

3. Total : $10 + $20 = $30 <<< shown on 3rd text field

I need to filter 1st text field to only get the number. If user enter $10, I should get rid of the $ sign or if user enter any characters other then numbers (including

decimal) my program should be able to only extract the number (and decimal)

When user select the drop down, I submit the form to self, calculate this 2 numbers and show the result in the 3rd text field.

I tried to use ReplaceNoCase but it generates error since it doesn't like the empty value on the 2nd string attribute:

#ReplaceNoCase(Form.FirstTextBox, $, ,ALL)# >>>> this generates error because it is trying t replace it with a blank space..please help!