Buying a house in Sydney or Melbourne, if you pay the median price and get a big mortgage, could end up costing a lot more than you think.

Research by comparison website finder.com.au shows that a home-owner with a mortgage of at least $489,300 will end up paying $1 million over the duration of a 30-year loan, using the average variable interest rate of about 5.5 per cent. Any property priced above $611,625 will end up costing the…