This depends on the situation. Right before the olympics when visas were harder to obtain, agencies charged up to 10,000 rmb for a 6 month f visa. I think at the moment you should be able to get an f visa for under 2000rmb although they're more reluctant to give out 6 month ones these days. If you're unlucky you'll just get a 3 month one, or 6 month that requires you to leave the country every 30 days.

Don't pay an agent anything. Just go to entry/exit office yourself. The agencies want you to believe that there is some complicated procedure that a foreigner is unable to complete. It's not true, the paperwork is English and with a little research you can easily find out what all the necessary paperwork is.

@Bisis, it's not only a matter of complicate procedure or language problem. Depending on the situation, you cannot manage to get all the documents you want by yourself....

Agencies will sometimes get local connections inside the bureaucracy and help you to get a visa you wouldn't be able to get otherwise.

I got a friend who a couple of years ago went through this kind of agency to get a F visa, because he couldn't do it through the normal "entry/exit" channel, and he got his visa issued in Shandong, while living in Shanghai.

As for visa in Shanghai, the worst hassle is to be a non-working foreigner and married to a non-Shanghainese woman......that's really the hell to renew my visa each year....