We started with a few items that were all for sale in multiples of ten: $.30, $.80, etc. The students were given $1 worth of dimes, and they had to count out the correct number. They mastered this very quickly and had soon moved on to buying multiple items ... which meant they had to add the prices together and determine whether they had enough money to "purchase" them. When the kiddos were ready, we moved on to more complicated coin combinations ... $.37, for example.
They loved it, and they still ask if we can play store!