I see this question getting asked on message boards a lot. It's a perfectly valid question and I'm not sure why it's so hard to find answers with actual numbers. There are lots of people coming up from student/low-no film markets into a more commercial markets (just like I did) who are very likely to greatly undercut established rates without knowing any better (just like I did).
Union sound effects editors in LA get around 37$/hr according to this site
But keep in mind that these are for staff positions. Meaning they're getting a guaranteed number of hours every week, benefits, overtime, they're using studio equipment and software etc. Contract or freelance rates should always be at least double employee rates. At least.
Over on Gearslutz, the always generous Georgia posted her studio rates as of 2008
$150 / hour for music composition
$150 / hour for sound design
$150 / hour graphics
$200 / hour picture editorial
$225 / hour for ADR and dialogue cleanup
$300 / hour for Foley
$350 / hour for mixing
$350 / hour for printmastering and/or encoding Dolby or DTS
$450 / hour HD laybacks
$450 / hour VFX
This is for a full service studio of course and if you're a freelancer working in your bedroom, you may not think you can charge this much, but I know respected sound designers working out of home studios who are charging over $300/hr for sound design.
My current base rate is $80/hr but I'm considering raising it or charging on a strictly per-project basis. There are commercial projects that would happily pay much more and indie projects I'd love to work on who can't come close to paying that much.
Frequently for indie projects I'm really interested in, I'll offer a reduced rate. But I always make sure they know it's a reduced rate and I reflect that on the final invoice. There is a huge difference between commercial and indie film rates so it makes sense to do this, but if these low rates become the norm than we collectively run the risk of devaluing our work.
Anyway, my 4 cents (I'm a freelancer remember).