http://xkcd.com/287/

xkcd_order.py

def find_order(cur_order, menu, valid_orders):
    # Try every item in the menu.
    for selection in menu:
        new_order = cur_order + [selection]
        # Try again if the current item blows the budget.
        if sum(new_order) > 15.05:
            continue
        # If the order costs 15.05, then it meets our contraints.
        elif sum(new_order) == 15.05:
            valid_orders.append(new_order)
        # If money left in the budget add more by recursing
        elif sum(new_order) < 15.05:
            valid_orders = find_order(new_order, menu, valid_orders)
    # Exhausted all the branches here. Return valid orders
    return valid_orders

valid_orders = find_order([], [2.15,2.75,3.35,3.55,4.20,5.80,6.55], [])
# Remove duplicates
for order in valid_orders:
    order.sort()
no_dups = set()
for order in valid_orders:
    no_dups.add(tuple(order))
# Print orders
for order in no_dups:
    print order

Output:

(2.15, 2.15, 4.2, 6.55)
(2.15, 3.55, 3.55, 5.8)
(2.15, 2.15, 2.15, 2.15, 2.15, 2.15, 2.15)