This restaurant is too expensive. We can't afford to eat here.
We can afford lobster.
This makeup is very expensive. I can't afford it.
We can afford electricity.
The Chinese Zero To Hero YouTube video 动词+得/不+起 gives an idea of the conversion: basically, 不能[verb] = [verb]不起 and 能[verb] = [verb]得起. But then 吃不起 should mean 不能吃 = "I can't eat [something]" and 吃得起 should mean 能吃 = "I can eat [something]" (and likewise for 用). So I'm not following the logic here.
Question: How do 用不起 / 用得起 and 吃不起 / 吃得起 mean (un)affordable?