Price Difference Between <18 and 20 MF?

I got a pair of pants for with 20% mf (magic find). When searching the AH there are only a few pairs with similar stats (and with 20 mf). They all go for 30m+ buyout but as soon as i got to 18% mf (and lower) the prices shoot way down. is the extra 1 or 2% really worth it? is it that rare? What's the deal!?