Pick a reasonable number of years you believe the home will stand. Since many well-built homes last more than a century, a round number of 100 years is good.
Contact several flooring contractors and ask for bids on both slate and concrete floors. Include as much information as you can about how you want the floors to look. The cost of slate varies, depending on its quality and where it's from, so be sure you know what the contractor's source is. Concrete ranges from plain to highly decorated and custom colored, which subtracts slightly from its durability.
Compare the costs of the bids you receive. For example, if the bid on a 2,000-square-foot home is $28,740 for slate and $30,000 for concrete, you know that the floors amortized over a 100-year period will be about $287 versus $300 a year, respectively.