98 that's burning through gas extremely quickly

Today, I noticed that my girlfriend's 98 Cav (LS I think) automatic is burning through gas extremely quickly. I put in about 4 gallons worth today, and after about 30 miles of driving, all the gas I put in was about gone.

I'm not very familiar with cars in general, but my first thought was the fuel filter. At least, that's what I was told was the problem when my mom's LeBaron was burning through gas.

Anyway, what does the problem sound like and how hard/expensive would the potential problems be?

Thanks in advance.