The bottleneck was userland CPU, so probably the gzip processes. Took about 5 hours. Cutting of the path and piping through sort and uniq took another hour or so.
The only costs were the price/hour of running the EC2 instance. Network costs were 0 as you're only transferring data to your instance.
pv (pipe viewer, http://www.ivarch.com/programs/pv.shtml) and top are pretty handy to measure this kind of thing. You should be able to see exactly which process is using how much CPU, and what your throughput is.