(a) Fair market value of parts and labor charges. The Tax Collector may examine the reporting of all transactions covered by this Section to determine if an "arms-length" price is charged for the parts and materials. The applicable tax may not be avoided by pricing a part, which ordinarily sells to the customer at $10, at $5 and including the difference as "service" or "labor". In the absence of satisfactory evidence supplied by the taxpayer as to industry or business practice, the Tax Collector may use the cost of the part or materials to the taxpayer marked up by a reasonable profit, to estimate the gross income subject to tax.