I suspect that the exact cost of manufacture would be considered a trade secret or commericialy sensitive information, not to be divluged to potential competitors.
An approximate cost could be infered from discount lamp suppliers and wholesalers, they obviously make a profit, therefore the cost of production must be less than the sale price.
(remember that reduntant or obsolete stock might be sold at less than the cost of manufacture)
In some cases even the manufactuerer may not know the exact cost of production. For example if a factory makes many different lamp types, the fixed costs such as business rates, premises maintenance, payroll admistration etc can not allways be allocated accuratly to the differing products.