They are relatively easy to obtain on eBay... I have the 1973 $1 in question (run of the mill prefix), bird series $2 and $10... Tried to get the $5, but prices were too high. Only get them if it's cheap enough to do so. As I thought they were cool novelties, I got them. Don't think I'd want to go nuts on it, and didn't want a full sheet.
Again, because they're easily found for the $1 and $2... Check eBay for prices. There are sheets of $1s going for not much over $100 bucks. So, I guess they ain't worth all that much yet. But, as people continue to clip the sheets apart, the sheets should increase in price as they would become more scarce?