It cost me ￡160 from John Lewis. Had I been organised, I could have got it direct from the manufacturers without paying VAT (since it's a recognised medical device for treating a genuine condition), however that'd mean waiting until Tuesday & I needed it NOW. Yesterday. Last week. Probably last month.
I don't know if it's the best lightbox in the world ever, but it's tiny, portable, & presumably low-energy because it has LEDs rather than bulbs. It seems to work. I'll see if anything better exists once my literature report is submitted & I have spare time to look.