Frugality is not just finding the best deal at the store, but properly utilizing what you have spent your money on.
Several years ago, I read a book in which the author stated that the correct amount of shampoo to use is about a quarter's width in the palm of your hand. The correct amount of conditioner to use is about a dime's width in the palm of your hand.
Using this little shampoo and conditioner does work! My hair is shorter now, but even when it was longer, it was still enough shampoo. Conditioner is for the ends of your hair, not your whole head, so that small amount is just right for all lengths of hair. If your hair is extremely oily, or very long, I'm sure you would have to adjust the amount of shampoo and conditioner to suit your needs.
Also, using less shampoo and conditioner means your hair will be shinier! It takes extra work to wash out large amounts of shampoo and conditioner, so I suppose that would mean less water used too! (Theoretically speaking, anyhow!)
My shampoo lasts for months on end. I only buy several bottles of shampoo a year.