I bought a low-end Mac mini last year, upgraded it with 16 GB memory from Crucial, installed a Samsung SSD with Fusion Drive and bingo: reasonably fast desktop

I bought a low-end Mac mini last year, upgraded it with 16 GB memory from Crucial, installed a Samsung SSD with Fusion Drive and bingo: reasonably fast desktop system for less than $900. I guess you can do it for $800 now. The stock 5400rpm drive is unusable, but it's an easy fix if you're willing to open the box. With Fusion Drive, even a small SSD will make a huge difference.