I do not have a really current Belgian OBp catalogue(2001).
If I compare to the 1987 catalogue prices Rod mentioned, and look at the 2001 prices they were reduced to only half.
I guess a nowadays catalogue value would be around 2-3 dollars.
they are on offer for 1 euro plus shipping. nowadays shipping/stamp costs are more then the actual item.
Above is for stamps with or without gum.