Depending on the thickness you can get by very inexpensively with either a scalpel (for thin leathers) or a simple utility / box knife with replaceable blades (for thicker leathers). That is where I would / did start. Eventually one could get a round knife, japanese skiver, or a multitude of other cutting implements. These would be used to cut pieces of leather all the way through. If the type of knife you're looking for is for carving, a decent swivel knife should run under $30.