I feel any sale over £150 would be good, the reason being that even if people wanted a high defintion lcd tv, they would still only search for lcd tv, i.e. high def is not the sale generating term, so the value of the domain is diminished.
If you have the time, a site explaining high def would gain traffic from those interested in the technology, and an affiliate site would probably make money, but it's not as efficient as targetting the more popular buying terms IMO.
Rgds