Trying to take two strings with values attached and subtract them

As shown, I have written this code and have assigned values for CASH and TOTAL. What I can not understand is why I get..... "Traceback (most recent call last): File "C:\Python27\Checkout Counter2.py", line 29, in change = cash - total TypeError: unsupported operand type(s) for -: 'str' and 'str'"

I've tried multiple ways to make this work, and I dont see any difference between that and when it finds the total.

 print "Welcome to the checkout counter!  How many items are you purchasing today?"
    #NOI is number of items
    NOI = int(raw_input())
    productlist = []
    pricelist=[]
    for counter in range(NOI):
        print"Please enter the name of product", counter+1 
        productlist.append(raw_input())    

    print"And how much does", productlist[len(productlist)-1], "cost?"
    pricelist.append(float(raw_input()))
    if pricelist[len(pricelist)-1] < 0:
        pricelist.pop()
        productlist.pop()
        len(productlist)-1
        len(pricelist)-1

print "Your order was:"
subtotal=0.00
for counter in range(NOI):
    print productlist[counter],
    print "$%0.2f" % pricelist[counter]
    subtotal += pricelist[counter]
    total = "$%0.2f" % float(subtotal + (subtotal * .09))
print "Your subtotal comes to", "$" + str(subtotal) + ".", " With 9% sales tax, your total is " + str(total) + "."
print "Please enter cash amount:"
cash = raw_input()
while True:
    change = cash - total
    if cash < total:
        print "You need to give more money to buy these items. Please try again."
    else:
        print "I owe you back", "$" + float(change)