My experience is that the home value is not too different.
It is more a matter of the condition of the septic system and well. If they are in good condition (tested and verified by someone that is qualified) or if the house is on city water they would be equal.
If there is a problem with the well or septic, work to get a estimate of the repair/replacement cost, and go to the owner with that information to negotiate their fixing it, or getting a price break.
It is also a great idea to see if the area is moving to city water. That can cost a bit.
Another thing I found while looking for my house, was that there is a mindset to having your own water and septic. An independence from the city to rely on them for water and sewage, and you are at their mercy for price and regulations (like in my area in the summer, you can't water your lawn or wash your car whenever you want).
Also, consider if you have well water, you may need a water softener and whole house filter to get the calcium and iron out. There is the cost of the equipment if it is not already in the house, and then the maintenance costs of salt and filters. You might say you can get by without this, but it makes the water much more usable and friendly. Consider it another part of the cost of your house.
Mom mentioned your heading in this direction!!!
While every area is different - the difference between city water/sewer and private well/septic properties in the twin cities is negligable. One thing for you to consider is the age of the well and septic and whether or not they will need to be replaced any time soon. I would be certain to have the septic system tested since they are fairly expensive when they are replaced.
Have your agent check into a couple of things for you as well.
1. When is the area slotted to be connected to city water and sewer (connection assessments can be very expensive)
2. What are the regulations for replacing a system if needed (this will determine some of the cost if the system needs to be replaced)
Homes with well water are most rural. As for values...usually a little more expensive homes have sewer. Most all homes here have city water. I can tell you from experience, though, that it costs more to have city water and sewer. It is the sewer that costs. You pay not only for incoming water, but also for outgoing.
Hope that helps.