To estimate your monthly sales you will have identified as far as possible how many valeting jobs you expect to do and at what price. From this you can work out approximately how much you will have to pay your main suppliers. Of course, the materials needed may vary a little from job to job, but you will always need supplies of bodywork cleaning products, polish, polishing cloths, upholstery cleaner, plastic trim cleaner, wheel and tyre cleaning products and so on.
Contact suppliers to find out cost prices, including VAT - try to obtain a current copy of their trade catalogue and price lists. Estimate the total cost of the items you expect to need each month. Enter the cost of your stock in the months in which you expect to pay for it.
Car valeting is a labour-intensive rather than a materials-intensive industry, so the cost of the materials you use is unlikely to amount to a very high percentage of your total sales.