Your cost for the property is your net cost, after all credits and adjustments. It doesn't really matter what the credit is for–repairs, or help with closing costs (you pay a higher price and they pay your closing costs), or something else. Your cost basis is your net purchase price. (You can also include some of the legal transactional costs of the purchase like a county transfer tax. See publication 523 for adjustments to basis, they apply to rentals as well as personal real estate.)
https://www.irs.gov/forms-pubs/about-publication-523
I don't think there is a black and white rule about "allocating" the cost between land and structures, other than you must accurately reflect the value of the land, and there is probably something in the regulations about your method being "reasonable."
I wouldn't think about allocating percentages, I would think about the whole net cost. For example, if the listed selling price is $250,000 and you are getting a seller credit of $20,000, you would just think about the net price being $230,000 and then determine how much of that is land. There's no set percentage or rule of thumb, it depends on market conditions in your community. The real estate agent may be able to suggest a figure.
