1996 plymouth voyager problem?

I have a 1996 plymouth voyager. I just bought it a year ago because I really need it. I paid 2000 for it. But since then have invested close to another 2000. It was running verry good until now. The gas was finishing too fast. I reallized it's one of the motors injectors or something lilke that, which is not ping the gas and causing a leak that smells strong. How can this problem be fixed? A mechanic wanted to charge us 100.oo just to come look at it. it has 240, 000 miles. Is it worth it to fix? What is the best thing to do? It's been well kept un til this that happened. Thanks for all your answers