How much weight do dry beans gain when rehydrated?

Most types of dried beans absorb a little under 1.5 times their own weight in water during cooking (ref: http://www.springerlink.com/content/m2p808873j7r276w/).

Based on this information, you need to use a factor of about 2.5 on the weights depending on whether you had cooked or dried beans. If a recipe says to use 100g of dried beans and you have cooked ones, use 250g of them. If the recipe says to use 100g of cooked beans, you need to start with about 40g only.