I've bought one of these things: A "Brushmate 4+". Cost was £12.71, including extra vapour wick from the local Dulux colour centre. Already used it for 4 days, and the brushes were as soft as the day they went in. It's already saved me half an hour's worth of brush cleaning, and having to take Clingfilm on site to wrap them up. The vapour's supposed to last up to 6 months, but I guess 3 is nearer the mark, but at £2.50 for a new wick, I think that's OK. Certainly it works very well, and I'd recommend it to others.
For the record, they do a Brushmate 20, which has a galvanised carrying box with two compartments (one for white, one for colours), uses fluid wicked from a bottle (rather than a disposable sachet), and holds up to 20 brushes. This one costs about 50 quid, though.