The prices for the ingredients are from Auchan in Cluj-Napoca, and the measurements are made by manually approximating, because container shapes aren't quite simple. I believe the most inaccurate product was the lettuce, since density, leaf area, usable leaf size, and shape all varied with the radius. I just guessed.
The picture on the right doesn't do my accuracy justice, since I only had one hand, but was included for fun. You can see the markers for "0" and "2", meaning how many sandwiches had been made when the ketchup inside was at that position.
The ingredients I used were lettuce, ketchup, ham, cheese, bread, and electricity - because I used a sandwich maker.
The most pointless to compute is the electricity, which means I'll dedicate extra effort to it. It involves unit conversion and adding the VAT (the most complicated mathematical operation involved in this analysis).
My sandwich maker uses 750 Watts, and one sandwich is done in 2min15s. Which means I use 0.028125 kWh per sandwich. Electrica Distribuție Transilvania Nord says a kWh of "active power" is 0.02207 RON. The "reactive power" is negligible here, since my sandwich maker has a very high yield (electricity to heat). However, with the VAT, that's 0.0273668 RON. Therefore, it costs me 7.7*10^-4 RON to cook one sandwich. That value is invisible on the graph, when compared to the other ingredients:
The final cost is about 2.3 RON, if you pick either the cheese or the dill cream. If you're planning to start a business, remember the rent and the salaries.
As expected, the most expensive item is the ham. Become a vegetarian and save money, your health, and the world! Or perhaps a partial vegetarian if meat is too tasty. Also, don't take me as a role model. I don't always practice what I preach, try as I might.
Also, you're extremely unlikely to enjoy the same ingredients in the same proportions as me, so your results will vary. They shouldn't vary that much, though. If you do such an analysis yourself, feel encouraged to post your results!
You can view and play with the data here.
Update - 4/12: here's a pic:
Update - 2018-02-03: the cost revisited 5 years later!