Where can I find cheap car hire in Barcelona but I want to drop off in France?
Simple answer: you can’t do it cheaply, Car hire in Europe is expensive anyway (except on some of the popular holiday islands). One way rentals are always far more pricey and if you cross into another country it becomes even more so as then the car must be repatriated. Last time I drove from St.Emilion (near Bordeaux) to Madrid I found it cheaper to buy a fairly rough used car in Bordeaux and I abandoned it with the keys in Spain; if I’d had the time I could have sold it to a scrapyard. Cars in Spain are cheaper than France, so if you have the time you could buy a fairly good car in Barcelona and sell it at your destination. I won’t claim you’ll make a profit but you will minimise your costs. Best chances are if you buy a small hatchback and if it’s a French model it’s easier to resell as the French tend to be patriotic car buyers and are more likely to take a chance on a less-than-perfect example.