Niraj, the best solution would probably be to change the Key field of the pricing rates table to be a text concatenated field in the format obtained from the formula field of
List(":", [Category], [Department], [Service], [Assignment], [Pricing Level])
Then once you have that setup copy the values from the formula field to a data entry text text field using grid edit, and then set the key field of the prices table to be that text field. Maintain that going forward with a form rule which ensures that the text Key field is equal to the formula field. Make sure the formula field is one the form, even if it's hidden by a form rule so that the form rule can see the most update to date calculation of the formula.
Then on the Products record, make that same concatenation formula and lookup the Rates, in a Relationship and call the rate like [Rate(lookup)] and then make a snap shot field called [Rate] which will snapshot (freeze) that value so that your historical product records do not change retroactively.
