Looking for one to use on an RX7 running a GT42...

Why are the bigger brands so crazy expensive, £710 for a Turbosmart 60mm for example?
Are the cheaper ones like Demon Motorsport (£95 ebay) worth a look or is the quality so bad it's just not worth it? Do the valve's not seat 100%, or are the springs weak and don't hold boost well?

All experiences welcome!