correct me for using generators or tell me other way

I have A menu dict item as key and price as value. There may exist a combination of item that will be bit cheaper than single item. For exa:

menu = {
    ('burger',) : 5.00,
    ('pizza',) : 12.00,
    ('coke',) : 4.00,
    ('macpuff',) : 4.00,
    ('pasta',) : 3.00,
    ('french_fries',) : 2.00,
    ('burger', 'coke', 'french_fries') : 10.00,
    ('pizza', 'coke') : 15.00,
}

Now suppose i ordered few items then output will be min-amount of given order:

I/P >  burger, coke 
O/P >  9      (5.00 + 4.00) 

I/P >  burger, coke, french_fries
O/P >  10.00 

I/P >  pizza, coke, french_fries
O/P >  17.00    (15.00 + 2.00)

here is the code i tried so for all prices which i will use as generators:

def isSubset(a, b):
    """
        compare two iterable and return true if first is subset of second
    """
    b = list(b)
    if not hasattr(a, '__iter__'):
        a = [a]
    for each in a: 
        try:
            b.remove(each)
        except ValueError:
            return False
    return True

def rest_min_price(order):
    if order:
        for item, price in menu.iteritems():
            if isSubset(order[0], item):
                new_order = order[1:]
                for itm in item:
                    try:
                        new_order.remove(itm)
                    except ValueError:
                        pass
                yield price + rest_min_price(new_order)

but when i run this it's saying type error:

for each in rest_min_price(order_item):
    print each

TypeError: unsupported operand type(s) for +: 'int' and 'generator'