How do 用不起 / 用得起 and 吃不起 / 吃得起 mean (un)affordable? Among potential complements are some ending with 得起.  Some examples (from here and here) are:

这家餐厅太贵了，我们吃不起。
This restaurant is too expensive. We can't afford to eat here.
我们吃得起龙虾。
We can afford lobster.


这些化妆品很贵，我用不起。
This makeup is very expensive. I can't afford it.
我们用得起电。
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 = &quot;I can't eat [something]&quot; and 吃得起 should mean 能吃 = &quot;I can eat [something]&quot; (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?
In China, we more of use this as a fixed phrase for (cannot) afford. For example, we would use 吃不起 or 用不起 or 买不起 to show that we cannot afford to eat/use/buy something. The simple logic is this:
-得起 means that you can afford to do an action
-不起 means that you cannot afford to do an action
This is probably a fixed phrase you need to memorize.
P.S. When we use -得起 or -不起, we do not necessarily mean afford in the monetary context. For example, we can say 我承担不起这个后果 to mean &quot;I cannot bear (承担) this consequence (后果)&quot;.