Maybe not for you ;) The rest of us usually need a few weeks to prepare for these algorithms-heavy interviews. I'd rather know in advance if it's worth the trouble!
It's worth it. Don't worry, it took me weeks too. It really helps if you are the kind of person who actually likes doing the dumb algorithm exercises. I accidentally did something clever by interviewing at a handful of small companies I wasn't very interested in first, so I got to warm up the interview-question region of my brain.
The averages being, in my area:
SDE £61,668
SDE III £64,190
Senior SDE £84,307
The salaries discussed in these comments are only valid for San Francisco / Seattle / New York. London has incredibly bad compensation for programmers, relative to the cost of living.
I spent every weekend for 2 months preparing. Best investment of time I've ever spent in my life.