Both processes produce water with very little dissolved content and they charge you in completely different currencies. Distillation costs energy, at a rate set by the volume you make. Deionization costs resin, at a rate set by how much dissolved material the resin has to remove. That difference decides which is cheaper for a given household or workshop, and it can go either way.
Distillation: energy per volume
A still boils every drop it produces and condenses the vapour, which means the electricity cost scales directly with output and barely at all with the water's quality: hard water and soft water cost much the same to distil. Against that, hard water leaves scale in the boiling chamber, so descaling becomes a routine chore and a neglected still loses efficiency. Production is slow, measured in hours per batch, so a household needing volume will be running it constantly.
Deionization: resin per dissolved load
A DI cartridge is consumed by the total ionic content it removes, so the cost per volume scales directly with how much is dissolved in the feed. On low mineral water a cartridge lasts a long time; on hard water it is used up quickly, and this is the single biggest variable in the economics. It uses almost no electricity and produces at full flow rather than in batches, which is why professional cleaners use it. The cartridge is the whole running cost.
The combination that beats both
Feeding a DI cartridge with reverse osmosis product water changes the arithmetic entirely, because the membrane removes the bulk of the dissolved load cheaply and the resin polishes what remains. A cartridge that would last weeks on tap water lasts many times longer behind a membrane, and the RO stage itself runs on line pressure. For anyone producing low mineral water regularly, that combination is usually the cheapest per volume and it is why it is the standard professional arrangement.
Buying it instead
For occasional use, buying distilled water by the bottle is cheaper than owning any equipment, and it is what most households do for a steam iron or a humidifier. The USGS describes hardness as dissolved calcium and magnesium, and it is those minerals that both processes remove and that decide how much either costs you. If the aim is drinking water rather than appliance water, neither is the right route: reverse osmosis with remineralization costs less and tastes better. Our softener guide covers hardness across the house.
Questions people ask about deionized water vs distilled water
Which is cheaper to produce?
It depends on your feed water. Distillation costs energy per volume; deionization costs resin per dissolved load, which rises sharply on hard water.
Why does hard water make DI expensive?
The resin is consumed by the total ionic content it removes, so more dissolved material means a cartridge is used up faster.
What is the cheapest arrangement for regular use?
Reverse osmosis followed by a DI cartridge, since the membrane removes the bulk cheaply and the resin only polishes what remains.
Should I buy or produce it?
For occasional appliance use, buying distilled water is cheaper than owning equipment. For drinking water, neither is the right route.