The cheapest are the NASA range with the Target wind @£139, the Clipper Wind @£184. The next in price appear to be the (Plastimo) Navman - that I have fitted - at £319. But hunt about as you might get them a bit cheaper. And no, the boat show isn't necessarily cheaper.
Irrespective of the instrument you choose, check the cost of replacement masthead parts. You will need to beware cormorants, etc. landing on your masthead light - there is an awful lot of flapping at the final moment!
I lost the wind direction vane from my new NASA Clipper installation after only two weeks and two of the anemometer cups as well, before the end of the same season.
The wire hoops I had put on top of the light as an anti-bird feature had been squashed flat (must have been an albatross!)
We now sport a bright red painted laminated fibreglass cone atop the masthead light (friendlier and less messy than a 6 inch nail), so even the little blighters as well as the big ones now leave the mast alone.