It does sounds low. However, I live in a pretty expensive area and regularly see jobs posted on craigslist in that range.
Wages are driven by supply and demand. Thanks to the ever-declining cost of equipment there are a lot of people out there with an HVX200 or equivalent and an editing system, probably more than there are jobs for them, so the appropriate wages will depend entirely on how low those people are willing to go and how many there are competing for available jobs. On the flip side there's a lot more video production being done than ever before - the end result is there are going to be a lot of production jobs that fall into a lower hourly rate.
In general you do get what you pay for - the more someone is willing to pay, the better they're likely to get (to a point, of course). That said when I see someone offering a low hourly wage I've found that often represents the quality they are willing to pay for. Many projects require nothing more than basic competency, and when that's the case there's not much worth in paying high wages for top-shelf talent.
The reality is $20 an hour is still a pretty decent wage for most jobs - it's well over double even our high local minimum wage. On a full-time annual basis it hits right about at the average wage for an individual in the US.
The one thing I'd be cautious of with any video job where the wages are on the low side is being very clear on what exactly that wage covers. If you're getting paid $20/hour, but you end up commuting an hour each way and aren't paid for that time your effective hourly rate just dropped to $16/hour. If you're meeting, or prepping equipment, or planning a shoot, etc - anything that's outside of the actual production time - and not accounting for & getting compensated for that time, then it reduces your effective hourly rate. If you're working on a purely freelance basis you also have to account for your own overhead - equipment, insurance, rent, etc, plus your time managing the business doing things like invoicing and other paperwork. If you don't account for all of this it's easy to have your effective hourly rate drop below minimum wage if your starting rate is too low.
Then again - even if your effective rate is only equivalent to minimum wage it might be worth it if your other option is working for minimum wage in a non-video related industry.
So only you can decide whether it's an appropriate wage for you - and that's going to depend on how much available time you have, how much experience, what other employment options you have, etc. It doesn't really matter how much any of the rest of us say you should charge if no one in your area is willing to pay that much and you need the work.
