I bought a solar charger like this to keep plugged into the 12v outlet on a van that I rarely use.  I have not had it sit for a super long time since I plugged it in, but it has sat for a week and it seemed to be a strong start.  Mine was $10 at Harbor Freight.  I think mine would be heavy for backpacking, maybe that is where the higher prices come in....lighter and smaller for backpacking.

I brought the box in to work to show a co-worker who has an extra vehicle that sits as well and it sparked a long conversation about how this technology has helped 3rd world nations where electricity isn't readily available but has a cell signal and how people buy these solar charges to charge cell  phones -vs- taking a 2 day trip into town to charge the cell phone battery and how these people can now communicate easier because of it and therefore have a better quality of life....it was a long conversation  ::)

I don't have a solar cell phone charger, but I would buy one if I was in your situation.  My car charger seems to do its job and I know others who have used them before.