Can you do a search and see what property is really going for in your area?
The assessment value is for taxes and may not be an accurate price point. Assessment seems to depend on how much money the state/county wants for their budget. Our place in Wyoming was assessed for $260,000 when we bought it, about 2 years later the county assessor came out and re-assessed all the houses in the neighborhood and everyones assessment went up around $23,000. Surprise, surprise! about 2 months later they proposed building a new school and see, they have the tax base to support it.
There are actually a lot of factors to take into consideration on the value of the lot.
Are you in the city limits?
Is that lot buildable? 0.77 acres is pretty small for a rural lot, most places require a minimum acreage. I think a lot of areas in WI require a minimum of 5 acres for building on a rural lot that will have a private septic.
How valuable is it to you to keep someone from building a house there?
What are the taxes on the place? Are you able to pay that increase in taxes every year?
If you are in the city and there is no septic in the street, then check to see if there is any plans for putting septic down that road in the near future ... any developments going in nearby or any larger tracts of vacant land that may be developed. Typically, property owners are assessed for the cost of installing utilities past their property. A lot of people don't realize that even if they do not connect up to it, they still have to pay for it.