Re: National Instruments GPIB-400
You can (and I think you should) build one for $20 and be done with it http://dangerousprototypes.com/blog/2014/01/13/open-source-hardware-gpib-usb-adapter/ It will not be able to do what proper GPIB adapter can but it's not a problem - you don't need these features.
Few companies who built $100,000 instruments invested efforts and money into designing and making a contraption that would interface to clusters of these instruments (probably pushing $1m) and have the guts to sell this contraption for outrageous $500-$1000. I don't take it as a declaration of war. I am happy that somebody can afford to hand over $1000 to NI or Tek or HP so that they continue working on whatever they are working on.
It's fine, seriously.
Personally, I won't use Chinese knock-off to connect to $50k instrument and I won't buy $1000 adapter to use with 50 year old junk I got from a eBay. But this is just me.