low/high is very subjective. depends on a lot of stuff.
iPhone rates seem to top out at around $200/hr, and iPhone devs "feel" like the hardest consultants to hire right now, so there's your range.
Note that these rates bear no resemblence at all to what a big firm would get. There are all sorts of other risks big firms mitigate that you can't. [%]
[%] (to wit: the risk of negotiating with your brain fully engaged)
You say iPhone devs top out at 200/hr. What do pen testers, (not necessarily at the Matasano level) top out at?