Why does a buyer value a dollar in a small purchase (e.g. TV) differently to a dollar in a large purchase (e.g. car)?
One explanation from psychology and behavioral economics is "mental accounting": people put different transactions into different mental accounts.

When buying a house, people are imaging (probably unconsciously) a bank account dedicated to that purpose. The account's balance is pretty large (including mortgage loans). \$5000 in or out of this account does not really have any significant impact on the account.

When buying a cellphone, people are imaging a separate bank account, of which the size is much smaller than the home purchasing account. \$500 in or out of this mental account makes a big difference.

If we pool all accounts together, then \$500 saved from buying a cellphone should be the same as the \$500 saved from buying a house.