Paint is expensive but you don't want to limit your creativity by worrying about the cost of materials. People like reasonably sized paintings though and galleries like huge ones. To jot down ideas it is great to use a sketch book and even better to add colour pencils into the mix. But I've found a great way to take it one step further into full colour mini prototypes.
Using high gloss ink jet printer paper in typical photo size, you can apply acrylics neat or as a wash to great affect. It takes the colour well and has a curious ability to permit you to slide the paint and scratch through it at various stages of dryness.
I have has good luck using a toothpick to apply and push paint about.
Since the size is so small, you don't use a lot of paint but it is in full colour and makes a great practice sketch. Use this to find ideas then later make a full size picture.