. >> for kathy nestor, it's become an obsession. >> oh, my god, $4.25 on the corner. >> reporter: finding the cheapest gas prices she can. >> the marathon down where my daughter goes to school is still $3.98. >> reporter: and then sharing it. she's a gas spotter, a mother of three from illinois and one of an army of volunteer who prowl the country and then post their real time finds on a website called gas buddies. >> reporter: is this almost a border line addiction? do you feel compelled to put the prices into your app? >> yeah, if you're going to define addiction like that, it's an addiction. it's a good thing. now it's $4.29 almost up a buck in a week. >> kathy said she saved her family around $400 last year at the pump. >> it doesn't sound like that, but it's a month worth of gas in the winter time. >> reporter: she's been doing this for four years, entering prices with an app on her phone. i have to admit, once you start, it's kind of hard to put down. >> i hit the price and that shows me what it was four hours ago. see, somebody just updated it. $4.25. >> reporter: i beat you to it.