Since there's virtually no drag between the planets, the distance doesn't linearly determine the amount of fuel. What matters is velocity changes. It's about 10

Since there's virtually no drag between the planets, the distance doesn't linearly determine the amount of fuel. What matters is velocity changes. It's about 10km/s to get into orbit, and 5km/s or so to transit to Mars. Coming back is a lot easier because Mars has less gravity and atmosphere, and you don't need to bring as much stuff back. So maybe let's call it 20km/s total? Fuel has mass, so it takes fuel to bring fuel. If you were to bring all the fuel you needed along, it's probably going to end up being 5-10x the fuel for getting just to low earth orbit. Either you build one giant rocket, or more realistically you launch a few smaller rockets. Alternatively, if you can manufacture fuel on Mars, you only need to take along 15km/s worth of fuel. A single falcon heavy launch could probably handle that. Said another way, such a mission could probably be completed with one or two falcon heavy launches. That's basically equivalent to 3-6 falcon 9's worth of fuel, and a falcon 9's fuel cost is something like $200k. So, $600k-$1200k in fuel. A spaceship system actually suitable for going to Mars would probably use cheaper fuel than falcon heavy. By the time it's actually built, though, inflation and politics will have made the fuel expensive again. So my best guess is a cool $1million!