Hi there! I figure I should lend my experience on Hankens. Basically a Hanken stands for "One of a Kind," and as been mentioned in previous posts were created by chief head animators of an animation project. There should only be ONE of this image created, hence the rarity and expensive cost. Not everyone can own a Hanken for the cover of an anime DVD or CD or art book so they tend to have a high value. I have been collecting cels since 2000 and I can tell you there has never been a time when Hanken are as "inexpensive" as they are now. There was a web site called Animaxis (now defunct) which featured many Hankens by AIC (Tenchi Muyo, Armitage, Etc.) and the average cost was about $3000 - $5000, most of which were art book inserts as covers had already sold. Other cel dealers connected to the then popular Anime Cels Ring (hosted by Animanga.com) would sell Hankens for about the same cost as well. Long story short - Hankens are expensive as if you have one or someone buys one you will likely never see that image ever again for sale as it would belong in someone's personal collection.
*Note: Rule of thumb, like a regular cel, the more sought out the image, the more detailed, the more popular a series the more expensive a Hanken (and cel) would sell for. There was this great website gallery with an excellent explanation of this (kimonocels.com now defunct) that I printed out and scanned that you can see from my website and link to photobucket (4 pages). It's a great write-up:
http://animationasart.rubberslug.com/gallery/inv_info.asp?ItemID=339719
Now as for post-production / Rilezu, I understand some are made in limited numbers to promote an anime. Rilezu as I understand are just copies that are hand painted (like the popular You're Under Arrest, Slayers and One Piece replicas). As for post-production I believe anime that were created CG only had some Hanken's made for advertising, just like an original Hanken. I am unsure, and please correct me if I'm wrong, but I believe there may only be one image created as well. I say this as I have a couple World of Narue Hankens that are huge (about 2 ft x 18 in) and beside the cels the backgrounds are hand-painted. So in truth "post-production" may be One-of-a-kind" Hankens if they are that, the only one produced and you can prove by comparison the image and painted background are exactly the same as the published image.
One more side note: I have purchased cels that were mass produced images (small like a regular cel but no punch holes for flipping machines) that were given out at events in Japan (like Huis Ten Bosch Japanese amusement park) and others that were mass produced and sold as "paint by numbers" cels in Japan for anime such as Sailor Moon and Galaxy Express. They have no production timing placement numbers at upper cel edges and the black line is "printed" on the cel. Those are all uncommon but not one of a kind.
Remember cel collecting is all about enjoying anime so cost is in the eye of the beholder. I hope my explanation helps! ^_^
John