The problem is you cannot say how much a promo is worth buyin it, cos a worth in something of a personal opinion.
If I go to my freinds i.,e. and say "Hey, you see this prodigy-promo - it cost me 25 €" they give a fuck if they not into the material or if they don´t see a worth in it.
A lot of people sell their promos for under 1 Euro cos they didn´t see something special in a promo, cos usually they´re not allowed for resale.
And sometimes a worth has something to do with the time. In the beginning the most Omen-Promos were sold for 45 €, now the price is 15 € and in 1 year nobody cares about the prices.
So if you don´t want to spend too much money - be patient and you will get - especially new promo material - cheaper in the future.
Okay, I must admit, there are some old things goin around which are absolutely rare and yes, they were expensive cos they are not often seen on ebay i.e. But I guess nobody really knows how much of a single / a promo really was printed. There are sometimes numbered items, sometimes it´s said there are i.e 500 copies available, but sometimes I have a doubt in these numbers.
I.e. Flint - Asteroids Pink Viny is limited to 3000 copies. I have two of them and it will not be difficult to get a 3rd one. And I know a lot of members here have this vinyl at home, lets say 300 members. But this is 10 % of all records available. Do you think, that will be realistic that ony BK-members have 10 % of an Prodigy-item ???

. I really don´t believe this.
IMO you should decide for your own, if something is worth the money. As far you can see something "cheap" in your opinion don´t wait and try to get your hands on it. Especially on ebay you sometimes can be the lucky winner cos you got an item really cheap....
EDIT: A list will be a good idea, but it´s hard to andle to keep it up to date