Have you tried Calumet?
I use an interfit softbox. I would say it was good value, works well, but is not designed or consructed for serious professional use. My softbox is fraying and the seams splitting, rods losing their shape etc, but then I've built and rebuilt the box 100+ times on location for different jobs.
It was really cheap so it works out at about £2.50 a shoot. The quality of light is really good and subjects can work under it happily without cooking. Low power draw as well, an issue if you also run a monitor and set light off of a single 13A circuit (with reds, you can light and nothing else)
That's the trade off. To get anything much better you would need to spend much more. Law of diminishing returns and all that. If you are going to work it really hard I would look elsewhere.