How do you value a residential buildable lot outside Madison WI?

[Mike Nelson]

Located in rural McFarland, WI outside Madison.  My mother is selling her home, and an adjacent, separate 1/2 acre buildable lot with access to water and sewer.  It is located on a rural street with about 12 homes.  A realtor recommends selling the lot together with the home, asking $225,000 rather than separately.  Also, the realtor bases the asking price of the lot on the village  property tax assessment of $30,000.  I think the value should be based on actual sales rather than assessed value.  Also, I think it may bring more money sold separately, rather than combined.  My understanding is that the village assessment is only a rough estimate of value and value is based on what a buyer would actually pay and sales of comparable properties.  How can I estimate what this lot is worth?  Any opinions?