Temporary root concealers (sprays and powders) and permanent at-home kits both cover gray regrowth, but they solve different problems at different price points. Whether one is cheaper than the other depends entirely on how often you use the concealer and whether you still get salon touch-ups on top.
What each product actually does
A permanent root touch-up kit - like Madison Reed Root Perfection (~$38, 2 applications) or dpHUE (~$37, 2 applications) - deposits permanent dye that bonds to the hair shaft. Coverage lasts 3-6 weeks and survives washing. One touch-up costs $18.50-$19.
A temporary concealer like L'Oreal Magic Root Cover Up (~$12.49 per can, about 30 uses) or Garnier Express Retouch (~$12.95) is a cosmetic coating that washes out in the next shampoo. It is not dye. It buys you a day or a weekend, not weeks of coverage. Cost per use is roughly $0.42-$0.43.
Annual cost comparison
At a 4-week cadence - 13 touch-ups per year - a permanent kit runs about $247 per year (using Madison Reed at $19/application). That is your complete cost if you are fully replacing salon visits.
A temporary concealer used twice a week year-round costs about $22 per year at $0.43 per use (104 uses at $0.43 = roughly $45 per year if you do the math on the full 52-week calendar; actual spend depends on how many cans you go through). But most people who use concealers are using them between permanent touch-ups or between salon visits, not as a full replacement. If you still pay $1,480 per year at the salon and add $45 in concealer on top, your total is $1,525 - more than a permanent kit would save you.
When a concealer makes sense
Concealers are the right tool when you have a few days left before a scheduled touch-up and your roots are showing at the part. They are also useful for spot coverage at the hairline for photos or events. At $12-$13 per can with roughly 30 uses, they are genuinely cheap on a per-use basis.
They are not a cost-effective replacement for dye if your goal is continuous gray coverage. A single can lasts about 15 weeks at two uses per week, costing roughly $45 per year. That is cheap - but the coverage washes out every time, so you are not actually covering your grays for those 15 weeks.
The cheapest overall approach
The lowest annual cost for consistent gray coverage is an at-home permanent kit used on a 4-6 week schedule, with a temporary concealer kept on hand for the last few days before your next touch-up. At roughly $247 per year for the permanent kit plus $20-$30 in concealer, your total stays well under $300 per year.
Compare that to salon touch-ups at $1,200-$1,800 per year (before tip), and the permanent kit-plus-concealer combination saves most people $900-$1,500 annually. Use the calculator above with your own frequency and salon price to get a number specific to your situation.