The going rate for a professional canvasser is 2x the monthly amount, I don't think I know of anyone professional that charges different.
You maybe able to get someone to do it cheaper, or paid hourly, such as the example of a guy from the local uni, but it doesn't necessarily mean that they will get compact work. If the guy isn't used to canvassing the chances are the work will not be compact as he'll miss opportunities to close the deal on the door and hence get less work that's spread about a bit.
You may drop lucky and find someone who is a natural on the door, but they'll have to be familiar with pricing, etc.
If you can get a confident guy though, be willing to train them up to price right and they're willing to work part time you could do well, though I doubt many would consider waiting for half of the payment 2-3 cleans down the line. I would also doubt offering one day per week would attract the person you're looking for.