The problem I have with that statement is, how do you know? How many of those people are walking around with a new iPhone? How many of them have nice new cars? How many of them are wearing brand name shoes and clothes?
Very often teachers and dealers will shy away from a product solely because they think it costs too much. However, the long term cost ends up being significantly less than all expectations. Especially if they have to buy one or two more of something to replace their initial purchase. There are two difficult pieces to overcoming the "they can't afford that" assumptions. One lies with the parent and one lies with the teacher/dealer.
First the teacher/dealer. The obstacle here is making assumptions. You must stop being a clerk just ringing up the widget for the customer. You have to become a salesperson (I will save my rant on this for another post). Ask questions to get answers. Don't assume anything! If you ask the right questions, you can find out everything you need to know and more (including what they can truly afford).
Now for the parent. The obstacle here is priorities. Most parents will put their child's needs ahead of their own desires. Parents will often sacrifice personal needs to meet the needs of their child. I am not saying all parents are this way, but the majority will put their child first. How important is it for Mom or Dad to get the new iPhone when they already have the previous model?
So, before you start slipping back into clerk mode and making the assumption that your customer can't afford something, put on your salesperson hat and start asking questions.
