First way :

1. your price field can be fields.function(_get_price_from_product...) (read documentation about it)
2. Add an on_change event in your module view on the product field (read also documentation about it)
3. This on_change function can call directly the _get_price_from_product function to return the good price.

Second way :

1. Your price field can be a fields.related('product_id', 'list_price', ...) (read doc)
2. I think depending your version, it will be re-calculated automaticaly when product_id change (to check). Else Do the same as previous 2. and 3..

Last way :

1. If you want to change price independently than the product price, price must be a fields.float()
2. In on_change method of product field, change the price

Good luck