Small businesses do purchase press-release writing services. For current fees, I would suggest checking with other writers in your local area and conducting an Internet search. We actually offer this service through StartupNation too.
I wish you all the best,
There are some small businesses that will hire freelance writers for press releases. I recommend going to freelance websites like Elance or ODesk to begin with. Here you will both find projects to apply for as well as get an idea of what others are charging for this type of service. The payment is really all over the board, so you will have to decide yourself what you think your services are worth.
Do you have PR experience already? Then you should be able to charge a bit more for your services. If you are just starting out, consider writing a few releases for clients at a lower wage. This will help you build up your portfolio to help you snag the bigger wages further on down the road.
Good luck in your endeavors.
I am sure small businesses with no PR agency would go to individual PR agents for a low cost solution. As for the price, PRpro's suggestion is good. You can also check sites like Fiverr.com for idea on the price or some other popular freelancing sites.