In reply to JoshOvki:
> Any ideas what you will use instead of solar? Seems like the only reliable portable energy available.
Define "available". Radioisotope thermoelectric generators have proven to be pretty portable and reliable
To the OP: I don't think your survey goes into enough detail. "How much are you willing to pay" depends very much on how much power the device will provide. £400 for 40 watts would be pretty good, £400 for 4 watts rather less so.
There are other things too, like the ability of the device to run unattended, and whether it requires hazardous materials in order to function, and whether it requires any maintenance during normal use, etc.