Since they need you, go high on your price and ask for the world just to see what happens. If you start out fair, they might take advantage.
You didn't say how long the run will be.
A sewer line needs a manhole every 500 feet or less in my area. If you have a manhole on your land, then they have an access point and will use it if and when they need to. If it's just the pipe crossing your land, it will be very unlikely that they will ever need to access your land for a pluged line. All work is normally done through the manholes.
Why should you have to pay for the sewer usage?
Charge them a fee for crossing your land, and make them pay for your sewage usage. You can even tap into the line at the manhole on "their" land.
Will city sewage increase your lands value for taxes?
Will selling them the rights increase your tax bracket adversly?
If you come to terms and agree on letting them do this, be sure to get a very large cleanup deposit. Have a lawyer write it up so that you have control over what to charge them to repair your land, clean up garbage and deal with anything they don't. Figure $100 an hour for your labor at a minimum. Be sure the money is held someplace that you can access it if you need to use it, and they cannot get to it if decide they want it back.
Large deposits make for cleaner jobs!!!!!
Good luck, it sounds like a good oportunity for you.
Eddie