Each consumer wants to buy value added apples, tomatoes which are good for health and have many vitamins, but how we can evaluate if the product is or not value added?
I really do not know of any specific index, but if you think that high value would be equivalent to product differentiation, you could establish that all the value on top of the price of the generic one is added value. You could compare both prices and eventually find out how the differential value is composed.
Another possible mechanism could be to analyze the cost structure of the so called High Value Product and identify its items. If knowledge, capital or entrepreneurial organization are responsible for an important proportion of the cost, you could infer (although not always) that is is a high value good.
Effective menera verify the added value of a product, is to relieve the supply chain of the product and assessing that part of the chain is. Verified this, you can develop a new link to add value to the product. In the example tomatoes, canning belongs to a next link, this is value added. In value terms of use, durability and adds more likely to save and transport in terms of economic value, adds a wrapper and standardized information.
Also you can add more value towards or ahead of the preparation for consumption: produce and cook the sauce, prior to canning. Thus we near the end of the production chain or value added.
I hope I have made some new addition to your question, good luck!
Value addition to an agricultural product depends upon the demand on utility of the product for various usages... That is where the Big picture is to be created for the product or service
Eg... The Chewing gum which is out of a RESIN...https://www.researchgate.net/publication/262772323_CHEWING_GUM_Manufacturing_Process_Flow
has a big picture as follows....https://www.researchgate.net/publication/262772432_AGRI_PRODUCTS_-TEREBINTH_MARKETING_BIG_PICTURE?ev=prf_pub
Article CHEWING GUM Manufacturing Process Flow
Data AGRI PRODUCTS -TEREBINTH MARKETING BIG PICTURE