What you are actually buying is time
Security software is sold as a term, and the code you buy adds months to an account or to an installation. That makes the headline price meaningless on its own: a licence at half the price for half the term is the same money, and a two-year listing next to a one-year listing is not a bargain until you have done the division.
Terms are the field most often compressed in a listing title. Where the length is not stated, assume the shortest the vendor sells. Where a listing quotes a monthly figure, multiply it out — subscription software is one of the few categories where the per-month framing is used specifically because the annual number is less attractive.
Device counts and what counts as a device
Consumer security suites are licensed for a number of devices — commonly one, three, five or ten — and phones and tablets usually count. A household with two laptops, two phones and a tablet needs five, and a one-device licence bought by mistake cannot be extended by buying a second copy of the same thing; the two do not combine.
Check whether the count is per device or per installation, and whether the licence covers the platforms you need. Several vendors sell a Windows-only tier alongside a multi-platform one at a similar price, and the difference is easy to miss when the product names differ by a single word.
The renewal price is not the purchase price
The introductory price and the renewal price in this category are frequently very different, and the renewal is usually automatic. That is the business model rather than a trick, but it does mean the cheap first year is a genuine saving only if you diarise the renewal date and decide again.
Turn auto-renewal off after activating unless you actively want it. You keep the term you paid for either way, and you convert a silent annual charge into a decision you get to make. Vendors generally price a returning customer who asks about renewal considerably better than one who simply lets it run.
New-customer-only and regional keys
Some codes are issued for new activations only and will not extend an account that already holds a licence for the same product. The listing does not always say so, and the failure looks like a bad key rather than a licence rule.
Regional restrictions exist here too, though they are less consistently enforced than on games. A licence issued for one market can refuse to activate an account registered elsewhere, and support will not usually move it.
Switching vendors without breaking the machine
Uninstall the previous suite completely before activating a new one. Two real-time scanners on one machine is the classic cause of a licence that "does not work" when the licence is fine and the software is fighting — each treats the other as something to be blocked.
Most vendors publish a dedicated removal tool for exactly this, because their own uninstaller routinely leaves drivers behind. Running it, rebooting, and only then entering the new key turns an afternoon of confusing symptoms into a five-minute job.
What the higher tiers actually add
Vendors sell the same detection engine across their range. What separates the tiers is bundled extras — a password manager, a VPN with a data cap, identity monitoring, parental controls, a firewall replacing the one already in the operating system.
Judge those on their own merits rather than as security. A bundled VPN with a small daily allowance is not a substitute for a VPN subscription, and a bundled password manager is worth having only if you will actually move your passwords into it. If you would not buy the extra separately, the tier that includes it is not a discount on it.
The detection quality question is answerable independently. AV-TEST, AV-Comparatives and SE Labs publish comparative results regularly, covering the engine rather than the bundle — which means the score applies across a vendor's tiers, and the cheapest tier from a well-scoring vendor detects what the most expensive one does.
Performance, and the cost that is not money
Security suites sit in the path of everything the machine does, and they differ measurably in how much they slow it down. The same labs that publish detection results also publish performance impact, which is the number worth reading on an older or lower-powered machine.
The heavier suites announce themselves in ways that are easy to mistake for a failing computer: slow application launches, long boot times, file copies that stall. If a machine became sluggish around the time a suite was installed, that is worth testing before assuming the hardware is finished.
Bundled extras contribute to this as much as the scanner does. A suite installing a browser extension, a VPN client, a password manager and a system optimiser is four background components you did not separately choose, and most of them can be declined during installation without weakening the protection you actually bought.
Before you buy
Confirm the term, the device count, the platforms and whether the code is new-customer-only. Work out the renewal price before you commit to the introductory one, and turn auto-renewal off once you have activated.
Free tiers from reputable vendors, and the protection built into a current operating system, are considerably better than they were a decade ago. A paid suite is a reasonable purchase for the extra features, but it is worth knowing you are buying features rather than the difference between protected and unprotected.