Roger Sessions has a great post on the cost of IT failure. His estimate, including direct costs and lost opportunity costs, is USD 6.18 Trillion. Almost enough to make a banker interested.
How much of this could have been prevented by applying Human Centred Design , usability assurance? Perhaps half?
For the UK, his figures (in B USD) are
Cost of IT failure 200
If usability represented about 10% of IT spend, then in the UK it would be a USD 6B business. So, at say 1.5 dollars to the pound, and GBP 150k per usability person (including overheads etc.) that would be 28,000 usability people in the UK. Some room for growth there, I suspect.
His numbers come from WITSA. I didn't find the document. A google search of the site found 1 reference to the word usability (not a particularly focused reference) and none to ergonomics.