The percentage of Amazon's cost to selling price is a business decision that depends on many factors, including the type of product, market competition, target profit margin, and consumer price sensitivity to the product. This ratio is often referred to as the "gross profit margin."

Generally speaking, gross profit margins can vary greatly between industries and products. Some products may have lower gross profit margins because the industry is highly competitive or the product is expensive to manufacture, while other products may have higher gross profit margins.

However, the right percentage of cost to selling price should enable you to meet operating costs, profit targets, and remain competitive. You need to consider the following factors:

1. Cost structure: Understand the manufacturing costs, logistics costs, packaging costs, marketing costs, and other related expenses of the product. This will help you determine the required selling price level.

2. Market demand: Understand the price sensitivity of consumers in your target market to the product. If the market is price sensitive, you may need to reduce the percentage of cost to selling price to offer competitive prices.

3. Profit target: Based on your business model and goals, decide the profit margin you want to achieve. This will affect your ability to set selling prices.

4. Competitive environment: Understand your competitors’ pricing strategies and market share so you can decide how to price to remain competitive.

Finally, taking the above factors into account, you can develop an appropriate cost-to-sale ratio and test and adjust it in the market. Based on the feedback and data, you can gradually optimize your pricing strategy to achieve your desired profit goals.