You can look up benchmarks. It's different depending on the model of Mac Mini and Model of LLM.

The take away is that some of the Apple hardware hits a sweet spot for performance and price which may change in the future but for now it's causing a lot of demand so people can run inference without GPUs.

Also Macs keep a lot of their resale value so you can use them for a while and then sell them for sometimes 80% of their original value.