This is for the actual ETA. For the weight you need to calculate everything into one unit (except if you do multi modal routing). And you could choose € or $ but the most natural seems to be time as you always can ask “how worth is this detour?”

As fuel cost is proportional to distance in your case you could already use the ShortFastestWeighting (distance_factor) as it controls the time you want to trade with distance (assume a route that takes 1000sec and is 10km long, then a value of 0.03 (30sec/1km) means that you would like to drive maximum 11km to reduce the travel time to 970sec or 12km to reduce it to 940sec.)

And you can do the same for toll cost (also time vs. money) and for any other priority you mentioned.

Why is this unrealistic? This seems like one of the classical weighting options .

People think this is the classical weighting option, when actually they mean “shortest regarding time” not distance. Distance is a very weak measurement, especially for car. But also for bike & walking. Assume a walking route from A to B with a steep mountain in-between and you’ll save 1% of the total distance. Nobody would do this, everyone would go around the hill and arrive faster and invest less energy, i.e. “shortest”