I'd expect it to provide about the same value as a book, and be browseable; not something that is purely linear one after the other (e.g., I could skip to dynamic programming, graph algorithms, or whatever topics there happen to be). I think that a basic interview puzzle book should be in the range of $25-$40, and depending on that price point be from 150 to 300 pages, with roughly 50-100 problems.

Though, admittedly, I wouldn't pay for this at the moment because I'm happily employed. I did spend about $100 on books to brush up when I was interviewing in late 2011. I found actually the best practice for interviewing was going on interviews (I ended up interviewing at around dozen companies over the span of a month after I was laid off).