Does a finished basement typically contribute to the advertised square footage of a home, or does it vary by location?
Just look at similar homes in the neighborhood and look at how they are marketed. I have seen homes with 1000 sq.ft on each level finished being marketed as 2000 sq.ft and also 1000 sq.ft. Many times it is marketed as 2000 sq.ft when the basement finishing is not permitted, and 3000 sq.ft when it was done by the builder. Either way just be aware of the $/sq.ft pricing. It is useless because of how the area is counted. i.e If the above home is $300K, the price /sq.ft could be 150$ or 100$ (meaningless).
It can contribute to the overall value if supported by local market data, but it cannot be considered as part of the square footage.
The value added by basements varies according to your specific market. Examine the sales of similar-aged houses in your area with comparable above-grade square footage to see if those with basements sell for a higher price.