I can't really speak of price as i'm not on your side of the pond, but from a discussion with my boss about actually stocking the lower wattage amps and like the epi valve jr, the dark horse, the vox night train and AC4, we came to the conclusion that most people wouldn't want to pay for lower watts simply because they don't know how loud it could be and won't buy anything under 100w (overkill, i know...). and for the people who do want a home practice amp, it would be too loud anyway.
it would take a few good years (locally) of working the crowd to generate interest and buzz. now, i know a few players who actually know the fact that it's fun to have a lower wattage amp for home practice, but most of these seasonned players gig and actually have more incentive to buy gear for their gigging than for their home practice, because most of the time they play loud regularly. the concept is great, but i don't beleive it is viable unless you have the funding to back up the experiment.
i guess i'd pay around 500-600$ for such a head though, but i'd like it to switch between low power and ''quiet drummer'' power (like 1 to 5-10 watts, dual power tube design..), though at that point, maybe i'd consider building it myself for myself only. a fun project, and a good piece of gear to keep around :D