The prices change every 5 minutes for sales and such, but there's generally always a ton of 50 cents a watt solar panels on Amazon.

Permanent installs that work unattended are slightly scary and have to be done right so something doesn't leak or start a fire, but the portable folding panels are usually similarly cheap.

You could get one of those plus a charge controller and LiFePo4 for not too much.

Everything still needs to be done carefully though, and unfortunately almost all cheap charge controllers have a ton of idle power draw, they're kinda crappy compared to the tech built into a random cheap solar generator, I don't think you can make anything as nice as a commercial generator for cheap.

Most the DIY education kits for kids type stuff seem pretty crappy too.

An ESP32 module with onboard lithium charging plus a USB-C folding panel has a lot of possibilities though, if you have any projects ideas that could be done with such things.

Youtubing might be good for initial exploration, stuff maybe like (beginner-friendly dyi solar) https://youtu.be/adFGmOlDM-Y