Compare contract phone plans against prepaid options.
Contract plans bundle a subsidised handset into a higher monthly fee, so a fair comparison has to put the prepaid handset cost on the other side of the ledger. Totalling both routes over the same period is what reveals the real gap: prepaid usually wins on airtime but has to absorb the phone up front, and the longer the comparison period, the more the cheaper airtime dominates.
Two-route total cost
Contract total = monthly fee x months; prepaid total = prepaid monthly x months + handset cost
On airtime, usually. Over a short comparison window the up-front handset cost can still make the contract cheaper, which is why the period length matters.
Often insurance, roaming allowances, priority network access and a warranty channel. Those have value that pure cost comparison misses.
Many contracts keep charging the subsidised rate after the handset is paid off, which is when switching to prepaid produces the largest saving.