The backpacks with built in panels are still expensive because I'm guessing it's hard to find customers who want to be in direct sunlight long enough to get significant charge, while carrying a backpack.
https://www.multpl.com/us-median-income/table/by-year
Regardless of whether wages match inflation completely or how long they take to do so, the fact is that if solar panels cost the same in dollars as they did 10 years ago, the average person's wage buys more of them.
You could counter-argue that other things gone up by so much that despite this, people have less money to spare for solar panels. But given that solar panels are competing with some of those things, importantly fossil fuels, it's still important to be clear that they are getting cheaper - as the trend continues a solar panel pays for itself more and more quickly.