In general for assets with uneven cashflows valuation (or fair value pricing) is performed by calculating the NPV of the expected cashflows. You need to estimate the expected cashflows for the lifetime of the asset. In the case of music copyright in the UK this is 70 years after the death of the author (note this is just an example and doesn't cover all of the possible legal differences) so you may also need to estimate your life expectancy if you expect that the cashflows will be long lasting.
There are many statistical techniques (ARIMA, regression, guessing etc.) to estimate and predict future cashflows with one of the most historically accurate being simply to use the cashflow from the last period.
The problem is that the buyer will also be going through this process and will have (at least) a slightly different NPV to yours giving (at least) two different fair prices - one that you have estimated and one that they have. Neither of these prices will be the final price but they will form the basis of price negotiation.
Books have been written on how to do this sort of forecasting so it is beyond this answer to give a full treatment of this. For the simplest negotiations it may be enough to get a feel for what the NPV might be like and then make a bid or offer for the copyright. If the valuation is likely to be high enough to matter then I'd recommend specialist statisticians and negotiators.