The numbers will be relatively the same.
There are some variances (x color weighs more than Y, pearl, eyes, dipped vs non, etc). I used a frog with a pearl belly, eyes and dipped. It shouldn't get much heavier than that. This wouldn't yield an exact count, but should be in the ballpark.
It's easier to do it by weight because there are more known factors. I know how much the plastic weighs, I know how many ounces are in a pound, all I have to do is drop a bait on a scale and then do the math. The biggest variable at that point is the person making the baits, but this should let you know what a perfect number is, and then you can figure out what is an acceptable margin of scrap, error, etc.
You could also do it by volume, but it would take longer, involve liquid, an accurate way to measure liquid, etc. Check with your production guys at work. I'm sure they do everything by weight.
Jason