1999 Mercedes Benz ML320 Major Gas Leak

Noises problem
1999 Mercedes Benz ML320 Automatic 130000 miles

Recently, gas started to pour from under my ML320 and the car stalled. Every time I tried to crank it up, more gas starts to come out. As long as the car is off no gas comes out, but when I try to to crank it up, gas starts to come out again. It's not just a little leak, its ALOT of gas. And on top of that I finally gave in and decided to just go ahead and fill up my car. (It cost $74 to fill it up!) Then 2 days later, with still more than half a tank of gas, this happens. We tried to plug the hole that we thought the gas was coming from, but we were wrong because after we plugged the hole and tried to crank it up again, gas was still pouring out. I'm having it towed to a repair shop tomorrow. How much does a new gas tank cost for a ML320 or at least to plug the whole up?