Wow, you are right. I didn't realize how much it has dropped in price.
To answer your question though, I am not sure where they are measuring from. The best advice i can give it to look up reviews of the cards. In those reviews, they often show it next to other cards/components and maybe you can judge a length off of that.
Another idea: since the PCI-E length is standard, we can use it as a metric. Measure (or look up) the length of the PCI-E slot. Then find a straight on top or back shot of the card. Then count (as precisely as you can, with a ruler) how many PCI-E slots long it is and then you have a fairly accurate measurement for the length you are interested in.