Other questions to put this anecdote in perspective: Was that a list price or the result of a negotiation? What were the competing products in the UK at the time, and why would a bank choose Dell if those products where significantly cheaper?
There were plenty of competing products in the UK - I could have had a better mouse for cheaper by wandering down to PC World, down the road, and paying £10. But the corporate ordering process didn't allow me to get involved in that - all I could do was express the fact that I needed a mouse with a wheel.
Well, there you have it. If you are Dell and you secure that exclusive deal, you get to charge high margins on certain incidental products.
Of course, as has been pointed out by olefoo, to get a deal like that Dell probably had to give up a lot of margin on other products. And throw in support contracts, and expend a lot of sales time...
Low-margin? I used to work at a large bank, and I recall Dell charged them some £25 for a plain USB optical mouse with just 2 buttons and a mouse-wheel. I'm sure many businesses could do with these kinds of margins.