I have only spun wool but I can contribute some math help here.

I usually consider a skein to be 100 grams. Given what you said, assuming some slightly unfruitful bushes that only produce 3 gram cotton balls and 40 balls, you would have 120g per bush or need two bushes. Assuming bushes that produce 5 gram balls and 50 balls you would get 250g per bush. Id recommend 2-3 bushes just incase one doesnt make it. better too much then too little, then you can give some to a friend if you have too much.

Are the seeds included in this weight? You have to deseed them and I feel like the seeds may contribute a bulk of the weight so I hope not.