My girls had long dresses, so I asked them to wear silver shoes of their choosing, knowing they both already owned some. That way they didn't have to buy new shoes if they didn't want to.
I did the same thing KateM. I asked my girls to wear pale shoes of their choosing. They all got something they were comfortable in and they weren't seen very frequently as the dresses were long. As we had paid for their dresses, hair/makeup and jewellry we didn't think we needed to pay for their shoes too.