Return of bonus

Hi there, Im new here, and I'm writing because I need some help calculating the return of a bonus feature I am trying to develop.

Basically, the bonus consists on a screen, which have 32 items, and the user will be picking prizes until 3 stoppers appear, Then the bonus ends and the user collects all the prizes he discovered untill now.

the posibilites will be N1 Value1 prizes, N2 Value2 prizes, N3 Value3 prizes, N4 Value4 prizes, NB Blank prizes, and NS Stoppers,
Where N1 + N2 + N3 + N4 + NB + NS = 32

If the chance of hitting this bonus is CH,

how do I calculate the Average return of this feature?
I presumed that the calculation will be guided into founding the avg number of items collected before picking 3 stoppers, and the the average prizes for this number of stones, or is as easy as getting the max prize, the min prize, the total number of possible combinations and do some calculus with that?

Any help will be preciated.
Feel free to ask for more details if something is not clear enough.

Thanks in advance.

HPS.

1. calculate the probability of winning 0 to total number of prizes (including blank prizes). Well call these x_p
p= total number of prizes (including blank prizes)
s = number of stoppers
w = number of prizes won
t = total number of prizes and stoppers (32 in your case)
x_p=combin(p,w)*combin(s,2)*(s-2)/combin(t,w+2)/(t-2-w)
(note: if you change the number of stopper until ends to 4 ,replace the four 2's with 3's.)

2. calculate the average prize value.(a)
a=(sum of all prizes)/p

3. the return is the sum of each x_p * p *a
I put a spread sheet in google docs
Any questions, just ask.

thank you very much! I will put a hand on this right now!

I just want to reaffirm that miplet's calculations are correct.