Although it is final part of any business but it plays very important role

This doesn't make sense! if it's important it's not the final part.

I am not a shopkeeper but if I read your example and you work that way you are already bankrupt.
If you pay 15 for transportation (did you?) and you only add 15 to the prize you paid (how come transportation costs are not in it?) you will work for 0, you can not pay your rent, you have not clothing, you can not pay if something gets broke, you can not pay taxes and so on. If you are close to bankruptcy you better write down all your costs add an extra post for ? and one for salary (you are your own employer, one for your inventary/building and the total sum plus an extra % is what the item will cost. That's how it works.

In average the price difference between an item bought by a shop and it's selling price is 100 - 500% +

If only sell 5 cakes per day it's better not to start a bakery but do franchising which you can do from your home (save yourself the extra costs.

What is the price per cake if it comes to ingredients?
What is the price if you add the extra costs?
You gave the end price for 1 cake and state that if you sell it you have 25% profit. May I ask what happens if you only sell 100 cakes or 50 instead of 150?