There are three main groups of IP valuation methods: cost based, income-streams based (the most popular type is a percentage of income) and of market comparison. The least reliable is cost-based approach. The general issue with all of them is a lack of reliable information in the case of idiosyncratic IP rights. Still, on the most developed IP market there some data sets that will help you with the valuation (e. g. www.royaltysource.com). You always may refer to industrial standard in many sectors of economy, including entertainment industry.