They are dirt cheap. But since nobody opens those things, and even if they do there are an average of 23 flyers in each envelope, the rate of return is very low.
If you are considering a mailing, make up a postcard - the extra cost is worth it - with flashy colours and solid paper stock. Just be sure to REALLY target the zip codes. You can get demographic info from the US Postal Office for free.
You have my email addy if you want some help with this ...