Actionscript adding code help

hello there! im markanator13 and i need some help!

so heres the deal..
i have 1 input text that is called "earned" with a button next to it called "add_button"
another text field called "subtotal" with "$350" in it...
another text field with the instance name of "sofar"
and another called "amountleft"

heres what i want it to do..
you put in something like "5.00" into "earned" and click on the "add_button", this is what i have:

add_button.onPress = function(){
_root.earned.text += _root.sofar.text;
}

but it just adds a "$" the the input text "earned"... and nothing to the text-field "sofar"

i dont know what to put so i can subtract the "sofar" from the "subtotal" to get the "amount left"
would it be:

getthatmoney function(){
_root.sofar.text - _root.subtotal.text = _root.amountleft;
}
getthatmoney();

it would be like that wouldnt it?
Can any help me?

##UPDATE##

ok i managed to get some hang of it.. i changed the first button code to this:
add_button.onPress = function(){
_root.sofar += _root.earned;
}

but now i just adds whatever numbers i out in aftereachother and doesn't add it in.
ex: 5.005.005.005.00
and not
20.00!!
Help!