This is back again. Cheapest is usually $2 per light.
This is great if you want to just quickly spruce up your garden/outdoor be it for an event, catch up with friends, etc. These things tend to not last too long. If you want longevity (and durability), you're better off with the wired ones.
I figured that these lights die because it looses the connection to the solar panel (too much heat?) not because of the components/batteries. If you know a bit of soldering, you can solder it back in-place and they should be good to go again after that.
We use these when camping, to glow around the tent ropes. Works really well.