Got mine last year on Amazon, was about ~$18 with two AA Eneloop cells.
Charging via USB, even from a desktop PC, takes a looonnnggg time. Can't imagine how long it'd take hooked up to even a quality, portable, solar panel. Although I love the idea, I gave up on solar panels for lightweight backpacking a long time ago. Spare cells are still lighter, even rechargeables. That said, placing a dummy cell in one slot will force twice the current into the remaining cell which is normally a no-no. But if using a solar panel, and if it can provide enough current, then that one cell will charge faster. YMMV...