Iâ€™ve sold homes (and had sales fall through) with wells and septic. Since Iâ€™m not first to the table, Iâ€™d start by saying the earlier posters seem to be right on.
First, you can have one and not the other, depending on the local public development of these systems.
Wells: If you have a well and it does not have a public system alternative, you can have two basic problems which will cause you to drill a new well or spend a deal of money to remedy. First is running out of water. In drought years this can be exacerbated. A deeper well may help, depending where you are. Obviously, North Jersey rock is more expensive to drill than South Jersey sand. The other problem is contamination or imbalanced mineral content. I had a buyer walk away from water that smelled like rotten eggs. (The seller had been using bottled water for cooking and drinking!) You can get water softeners and devices that add chemicals to clean up the well water, at a cost. If there is a hook-up to a public system available, it will cost you, as already indicated. This cost is very low compared to the value of the house and the proceeds from a sale, so itâ€™s a great item to negotiate the seller to cover. It ends everyoneâ€™s problem and, even if the deal falls through, the seller is left with a more saleable house (given that they can afford the fix before they get the income from the sale.) Iâ€™ve had sellers who have done just this and were very happy to accommodate the buyer in this fashion.
Septic. The costs can be astronomical. The whole issue is state regulated (very strict specifications.) but locally administered. Do not buy a septic system that is not ipse-pipsy squeaky clean. That wonderful patch of extra green grass in the back yard is being fertilized and you know how!
Having said all this, you have little choice in what kind of system you can have. If you want to live in X and X doesnâ€™t have public water and sewer, you have to settle for your own system that you will have to maintain. Forty thousand bucks over 20-30 years is only $2,000.00 a year. Alternatively, sewer bills can run pretty high but may be obscured by being part of the tax base.
The only two exceptions are: One, if a public system is available but not connected, in which case, get it connected by the seller and two, a private well or sewerage system, which do exist in this state. They can be quite serviceable but can also suffer from a lack of proper maintenance and then suffer condemnation. Fixes can run the clients of these system a ton of money because there is usually a small subscriber base over which to spread major costs.
Hope this helps.