How much does it usually cost to replace water line?

That's like saying " how much does a car cost?" it depends on what, where, how, why, how long, etc. You can get some good help here, from professionals, but they'll need more information like measurements...

Water line to what, how long, what materials, pro or handyman, etc...

In addition to all of the above tell us what area you live in. In my area we only have to go down 18 inches and can be dug by hand. However, up North where you have to stay below the frost line you might need digging machinery that will up the overall cost. As you can see many factors enter to the mix. You can't just say "one price fits all". If you wish to furnish with details, including local codes on water service materials and labor, (union or nonunion) costs, we may be able to come up with a ballpark figure. Regards, Tom