When you say bigger than 21", smaller than 25" do you mean only 22",23", and 24" or 21"-25". And also are you going with what the monitor says or viewable area, ie. a 21" monitor generally has a 19.8" viewable screen.
Overstock.com has a number of 21" monitors that are factory reconditioned for $200-$300 with a flat shipping rate of $2.95.