Not sure how well they work, but I've got the 2 pronged version, bought it from Wilkinsons last year. Seem to remember it being about £7, but not sure about that either.
What I will say is that the reading does vary from soil to soil, so it is measuring something ???. The 2 prong one also does light and moisture measurements though.
If you're gonna buy one, I'd get to Wilkos. :)