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;.