I've just been looking up some prices. In the below, the numbers are the benchmark scores on geekbench, following by pricing for RAM/SSD in parentheses.
i3-8100B (Mac Mini) 4687 13981 ($999 8/256, $1799 16/1024)
i5-8500B (Mac mini) 5159 20331 ($1099 8/256, $1899 16/1024)
i7-8700B (Mac Mini) 5667 24325 ($1299 8/256, $2099 16/1024)
What about NUC? You need to add RAM and storage e.g.
Crucial 8GB $40, 16GB $79
970 Evo Plus 250 $78, 1TB $228
Here are geekbench numbers and prices (from newegg):
i3-8109 (NUC8i3) 4349 8644 ($290 0/0, $408 8/256, $597 16/1024)
i5-8259 (NUC8i5) 4481 15755 ($427 0/0, $545 8/256, $734 16/1024)
i7-8559 (NUC8i7) 5206 17648 ($509 0/0, $727 8/256, $816 16/1024)
I guess you could say that in terms of the benchmarks, the i3 Mac mini is roughly equal to the i5 NUC, and the i5 Mac mini is roughly equal to the i7 NUC. I have the i5 Mac mini and it doesn't feel that fast. All the junk Apple forces you to run these days... (not as bad as Windows though I don't think)
The kicker is that the Mac mini has the SSD soldered to the motherboard. You can't upgrade it. [Update:] Whereas with the NUC, you can not only upgrade the M.2 SSD, you can also add a 2.5" SATA drive or SSD (as long as you get the "H" version).