Poor people can scrape up $15 if it means the difference between 6mo and 2mo. Sure there's some times when the purchase falls on a week between paydays and you buy the cheap (cheap up front, more expensive on a per day basis) version but the next time (or maybe the time after that) the purchase will need to be made at a more convenient time and you will have the extra $15 to spend on the one that has a lower overall cost.

Having experience with the $25 model is the barrier to entry, not the $15 difference in cost. It's not hard to justify $15 when the overall cost is lower and the expense is less than monthly. It's very hard to justify the $15 when you don't yet know if the product will last long enough to make the overall cost lower. And lets be honest here, products where +X% the cost gets you >X% the utility are microscopically rare compared to products where that is not the case.

They're poor, not stupid. They can do basic math and determine which product satisfies their needs more cheaply on an ongoing basis. For low dollar amounts