SalesLogix has a couple of numeric based custom text controls including currency, and numeric. One of the things you will find is that using these controls gives the user a slightly different experience based on the custom control chosen. One of the big differences is that the currency control enforces numeric input once the user leaves the field where the numeric control has no such validation.
var key = window.event.keyCode;
if ( (key > 47 && key < 58) || key == 8 || key == 127)
return; //It is valid so don’t do anything
window.event.returnValue = null; //Don’t allow the input