try this procedure:
1. based on your experience, estimate a standard % of mark up, say, 30%, 40%, 50%, etc.
2. now based on your estimate above, if your gross sales is P3,000 for the day, your mark up must either be P900, P1,200 or P1,500.
3. therefore, your cost of sales may either be P2,100, P1,800 or P1,500.
4. in the end, if you think that you are not being able to replace the merchandise that you sell with the same volume at the resulting cost estimate, you may try to:
a. adjust your mark-up estimate. otherwise,
b. check the replacement cost of your goods, especially in these times of rising prices.