We sent STD's to everyone, including our ginormous wedding party -- we even sent them to our parents who wanted to keep them as momentos. But we also made our own STD's so churning out some extra ones was not a big task or costly for us.
If you don't wanna spend the extra $$$, which is a significant difference, how about sending "cheaper" more simple versions? Or, if you find it appropriate, how about an STD email or e-card?