Should the hypothetical cost to connect to a public sewer include the disposition of the existing septic system? Would a "typical" jurisdiction require permits to remove a private septic system?
I have seen a wide range.
Generally, though, residential sewer tie-ins are something you can call the municipal engineer about, they usually prefer them to septic and might have programs to help the homeowner tie in that will assess the costs to the property to make it affordable. If they received EPA grants, they might even cover some of the costs. If they have an ongoing program they can even refer contractors and you can avoid a lot of red tape. If this is a newer municipality-backed utility installation; they may even have a fixed fee system in place.
If you are out in the suburban or exurban fringe, the sewers are sometimes designed for new subdivisions or commercial development, and the mains are not setup for small connections. Some sewer tie ins are inexpensive at several thousand dollars, others are tens of thousands. It depends on the type, size, and location of the lateral line. Do they have to go under the street, etc.
Also, in bigger cities, only certain contractors are licensed to operate in the right of way or on sewer, water, electric lines. You might have to hire a general contractor to orchestrate three or four different contractors: public engineer to draw for permits, digging the right of way, licensed septic installer to certify removal/demolition, tap-in plumber, and then a landscaper. Add into that if you need an environmental assessment before, during or after. Usually, anyone operating in the right of way requires a standing bond (typically a few million) with the municipality/utility.
Construction can get complicated. That is why the general contractors have the nicest trucks and offices and construction litigators, litigation engineers, and value diminution appraisers are in demand.