It is extremely dependent on your jurisdication, how the existing sewer lines are run to the septic, and how accessible the sewer cut in from the city is to septic system. I just helped a client buy a home here in Austin where the city had recently annexed the neighborhood (last 5 years). The neighborhood negotiated with the city as part of the annexation that they would not charge a tap fee for 5 years. After that time, the tap fee goes up on a sliding scale, $2,500 year 6, $3,500 year 7, $5,000 any time thereafter.
The seller had the sewer line run from the home to the street, no concrete cuts were required, and the installation cost was $3,500. This is for a home that was about 50' from the street on an 80' wide lot with adequate space between homes to get the excavating equipment in and out. The septic system itself just had to be drained and filled with a gravel/sand mixture. They did not have to take out the tanks, just fill them and seal them with a concrete plug on top. Total cost was less than $5,000.
But again, it can be much more expensive, as the first answer indicated.