Hi
@apricot.
If I understand your question correctly, then I can't see any way that you're going to find a magic percentage constant to help you.
However, the following formula will give you a marketing budget from your 30% profit requirement, ProductSalePrice and Costs. I started with your equation:
ProductSalePrice (p) - Costs (c) - MarketingBudget (m) = 0.3*ProductSalePrice (p)
... and did a little algebra to make MarketingBudget the subject. The formula is:
m = 0.7p - c
Examples:
m = 0.7(100) - 70 = 0 // Marketing budget: £0
m = 0.7(120) - 70 = 14 // Marketing budget: £14. Will the market tolerate a price of £120? Check out your competitors' prices. See [1] below.
m = 0.7(9) - 8 = -1.7 // Marketing budget: -£1.70. Not feasible.
m = 0.7(437) - 184 = 121.9 // Marketing budget: £121.90. Will the market tolerate a price of £437? Check out your competitors' prices. See [1] below.
[1] If your price is generally higher than that of your competitors, then would you be able to increase your costs a little in order to improve your product to make it better
value for money than that of your competitors?
Here's how I derived the formula from the information you gave:
ProductSalePrice (p) - Costs (c) - Marketing Budget (m) = 0.3*ProductSalePrice (p)
0.3p = p - c - m
0.3p + m = p - c
m = p - c - 0.3p
m = p - 0.3p - c
m = p(1 - 0.3) - c
m = 0.7p - c
As you can see, I simply added a marketing budget variable to the costs, then compressed the other costs into a single variable Costs to simplify the initial equation in order to make the algebra a little easier. I assumed you wanted a profit of 30% of the ProductSalePrice.
Perhaps one of your analytics apps has a similar calculator which might help you. If not, then on Google Play, I have an app consisting of various calculators, each of which take minimal input. If you (or anyone else) would like me to add this calculator to my app, then just send me a PM and I should be able to get it done this week. The calculator above would require you to provide only the ProductSalePrice and Costs, but other inputs are possible to make the formula more versatile.
You can check the formula with an intermediate equation .
Hope this helps.