The value of your house does not have any impact on the deductible amount of mortgage interest; it is the amount of indebtedness (and when it was incurred) that is relevant.
See, generally https://www.irs.gov/pub/irs-pdf/p936.pdf
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@mike_akane - the $750,000 threshhold is based on the mortgage amount and not based on the sales price of the home.
