Hi,
I'm surprised that it would be the case.
The donation plugin, which is preinstalled with HikaShop Business, actually uses that same event to use the value of a custom item field as price for the product in the cart.
So it does what you're trying to do.
So what that means is that the problem is probably with your code somewhere and not with the principle itself and that you're on the right track.
Now, without being able to check your code, it will be hard to say anything more precise to help you.