You'd have to live very, very poorly to not be able to save up a game budget or not have a bit of spare money. Maybe on minimum welfare that'd be a problem, but if you have ANY kind of job at all and you struggle to spend 15 $/€, you've probably got worse problems to consider than buying DLC.
No offense meant to you, I just find this argument a bit ridiculous. Maybe a student would have a problem with it, but students really aren't great consumers. But that's definetly the only group, apart from refugee's and people living who are starving, would have a problem saving up for or just buying something for 15 €/$ which they've known would be coming for several months.
And if you can't make that purchase even so, then it really isn't something you should blame the salesman for. Live within your means, within logic and reason.