There is currently no way to override the price using matrix fields or similar. You must enter modifiers separately for each product (unless they are “text modifiers”, but they cannot adjust the price as necessary). We are exploring ways to support it (while maintaining security and not allowing users to simply post their own price on the form).
This could be done using PHP. Someone already managed to do this in the allowance for bulk discounts - it seems that a matrix is used to store the values. I did not have the opportunity to see how this was done, but if you want to encode something to solve your problem, this will be a good starting point.
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