This year alone prices for solar panels have dropped by 30-40%. The biggest reason for prices not dropping for solar farms/large scale solar at the moment is that the supply of other equipment needed like transformers etc can't meet the demand with delivery times having reached 2+ years. So the prices for solar will likely drop another 40%-50% in the next 2-3 years at least in the rest of the world maybe not in the US because of trade wars/restrictions.