Collecting mechanical banks is a big deal. Lots of money moves around during big, highly attended, auctions. There are collectors out there that have been accumulating banks for centuries. One of the larger and more acclaimed collections was auctioned off a few years ago, bringing in 7.7 million dollars!
Mechanical banks became popular in the mid-century. Their original purpose was to get children interested in saving.
If you are interested in collecting mechanical banks the best place to start would be the Mechanical Bank Collectors of America, a super involved non-profit, with hundreds of members all devoted to the collection and preservation of mechanical banks. Their website offers a complete network for any collector-expert or novice. They meet yearly and even put out a triannual publication called the Mechanical Bank Journal.
The Kidd's Toy Museum in Oregon is the temporary site of the Mechanical Bank Collectors Library archives. If you would like information from any of their many documents and books, contact them. Also, your local library should have a book or two on cast iron banks; mechanical or still. Asking your librarian to help you find them is a good idea. I found these titles available on Amazon: The Official Guide To Mechanical Banks, and A Penny Saved: Still And Mechanical Banks.
Another element of mechanical bank collecting is obtaining mechanical bank trading cards. There are considered quite rare. Bruce T. Roberts penned "Mechanical Bank Trade Cards."
And here is the link to Dan Morphy's Mechanical Price Guide, with beautiful photographs and limited quantities it rings in around $125 a book.
There has been a lot of commentary about the images and offensive stereotypes in some of these antique banks. Below is a video from CBS Sunday Morning regarding such topic. It also shows a great variety of banks from the big Stekbeck auction!