Hi,
I'm not sure what you mean.
If you activate the "Display the cost per unit weight" setting of the Hikashop configuration and if you have a price and a weight in your products, HikaShop will automatically display the price per weight based on the main (the first one) weight unit in the "Weight symbols" setting of the HikaShop configuration.
So if you have the main weight unit as KG, you'll get the price per 1KG automatically added to the product page.
Is that what you're talking about ?
Now, you can also do your own display with a custom product field and a mass action. You can configure a mass action triggered on product creation / update with an action "update the values on the column of your custom field with the operation value:
100*price.price_value/product.product_weight
That should give you the price for 100g in the custom field whenever you save a product if the weight of the product is in grams.