If you're talking about your main sewer line from your house to the cities sewer main then the last time I had it done it cost me $150.00 for a plumbing company to run their machine down the sewer line and unclog it.
There was also an $80.00 service charge for them coming out so the total I paid was $230.00 to have the main sewer line snaked and unclogged.
For a drain line inside the house plumbers typically charge around $50.00 to $70.00 to snake the line.
Most times it's very easy to snake your own inside lines if you know where the clean out is and can get the clean out cap removed.
You can purchase a plumbers snake at hardware stores pretty cheap and they're easy to use.
You can also purchase or rent a sewer line machine to unclog the sewer line if you know what you're doing.