I have been collecting Betty Boop since the late 90s. During this time I have seen many items for sale and seen how much they can sometimes go for. There is no hard and fast rule as to price guides on Betty items but the rarer, older pieces definitely seem to attract the most money.
When I started collecting, I tried to buy things that are limited edition and also have the box/original packaging. Also, when scouring Ebay for a few years, I learnt what was selling well, and what was not, was was popular and what was pretty much tat.
After about 5 years of collecting, I decided to start selling items. I found there was a UK market for it. No where near as huge as the US, but still, some hardcore Betty Boop fans.
If you are thinking of collecting, I can highly recommend you try and buy a book by Leonard Ellis called "The Definitive Guide to Betty Boop Memorablia" as this will get you started.
You may wish to collect only certain items.,i.e. watches, dolls or ornaments for instance. I collected just about everything. From keyrings, pins, dolls, soft toys, 1930's Japan items, postcards, the list is endless, as is my collecting so it seems :)