Enter Amount In $ Validation

i have a textbox that has a value $6.00 for example the user has
to enter a dollar value which has to be less then $6.00 so for example if the user enters 4.00 an alert should be fired telling him he has forgotten his dollar sign ("$") then when he enters it properly like this $5.00 i have to check the value is less then $6.00 and it has to be done in javascript

the textbox which holds the $6.00 value is called saleprice
the textbox which holds the $5.00 or value less then $6.00 is called userprice