Its kind of like asking why some movie cels are 
more expensive. Since a series usually has a big 
run, and there are few OVAs, the ovas are rarer 
and thus more valuable, especially if the oavs 
are as good as the series (in some series, the 
oavs aren't as good, or the series isn't as 
good).  In most ova's the quality is usually 
better too, because it was a 'special' and didn't 
have as much of a deadline as a tv show.