I actually know Derek in real life (although not very well). He gave away most of his wealth, but he still draws a very comfortable salary from the fund he set up with the wealth that remains. He's rich.
I'm not sure where you heard the $1k/month figure, but there's no way he possibly makes that little. He lives in Oxford, possibly the most expensive city in the UK outside of London. $1k/month in Oxford isn't enough to rent a one-bedroom flat, without even considering other living expenses - there's no way he lives in Oxford on an income that low.