convert a JavaScript string variable to decimal/money

You can also use the Number constructor/function (no need for a radix and usable for both integers and floats):

Number('09'); /=> 9
Number('09.0987'); /=> 9.0987

Alternatively like Andy E said in the comments you can use + for conversion

+'09'; /=> 9
+'09.0987'; /=> 9.0987

Yes -- parseFloat.

parseFloat(document.getElementById(amtid4).innerHTML);

For formatting numbers, use toFixed:

var num = parseFloat(document.getElementById(amtid4).innerHTML).toFixed(2);

num is now a string with the number formatted with two decimal places.