Please remember that
you get what you pay for. You pay $25.00 for a 6" 250 CFM inline fan you will be getting a punk ass duct booster you could have not waited for just by going to home depot. These fans are for use in combo with a 250 CFM fan to boost the air through the ducting. I been there done that! Just look at the prices of other fans that are 250 rated CFM's or greater and you will start to wonder why this one is so cheap.
On the other hand, dont be afraid to spena little money on a fan. Shit, start to save for it!
These fans are very quiet and reated higher than the duct boosters.