The price of solar panels are evolving with a learning rate of around 22% per year.
The term learning rate in this context refers to the cost reductions that are realised as a technology becomes more mature.
The effect of the learning rate on cost is often illustrated with an experience curve like the one Seb Parquet has shown in his answer.
The experience curve shows the reduction in cost achieved with each doubling of installed capacity. If the cumulative deployment of a technology doubles, the learning rate represents the achieved reduction in costs.
The PV industry has good historical data with a relatively well established learning rate that allows future costs to be estimated.
One important point is to note that this all relates to the cost of panels only. When you are trying to establish the cost of energy from PV systems, you need to consider the balance of system costs including installation, racks, and inverters.
Also, even with well established PV panel costs and balance of system costs, its still not straightforward to compare the cost of energy from PV with conventional energy sources (i.e. to answer the questions about ‘grid parity’).