You can buy solar cells off aliexpress at ~$0.33 / watt (this is the lowest price atm). The most efficient cells are at 23-25% efficiency, but these are a bit more expensive (closer to $0.5).

Here's the first example I could find (https://www.aliexpress.com/item/10-Pcs-17-6-125-x-125MM-Mono...).