It seems like both are about $500 on Ebay, does that sound right?
One thing I can think of that makes 2012 Mac Pro better than 2018 Mac Mini is that 2012 Mac Pro has RAM capacity of 95 GB, whereas you can only go up to 64 GB on 2018 Mac mini.
But I would imagine mini would haver a processor that is newer and better than 2012 Mac Pro.
It depends. If you got Mini 2018 Core i3, which is about $500 the used going price, then the multi-core performance would be similar to a 6 core single tray Westmere Xeon in the 2012 or the 8 core Nehalem dual tray Xeon in the 2009 updated 2010. If you shop carefully for SSDs and HDs, you can raid both SSDs and both HDs to achieve disk I/O very similar to the Core i3. To match and exceed the Core i3 NVMe internal blade drive, you can install a NVMe blade drive inside the Mac Pro 5,1 (2012 or 2009 updated to 5,1). The advantage is that the Mac Pro, as you said, takes much more RAM and they are much cheaper DDR 3 1333Mhz kind and you can use non-ECC RAM, which is plentiful. You just can't mix ECC with non-ECC. The max OS it will support is Mojave and it has 4 PCIe slots. I installed a RX580 for video editing as well as for photography like the Topaz AI products and DXO Photolab, both of which do take advantage of the installed GPU, all CPU cores and lots of RAM. So for a 11 year old machine, it can match the latest Core i3 machine pretty well. But when you do go into the Core i5 and i7 series of the 2018 is when these Minis will start to shine and beat the Mac Pros, but for me, I figure that I really do not want to sink anymore money into Apple until I get a clear view of Apple plans to do with the ARM chip. My memory of myself gotten burned during the PowerPC transition to Intel taught a very good lesson in patience.
The only advantage that any 2018 or 2020 Mini has over the Mac Pro is Quicksync and the T2 chip which the Mac Pro don't have. If you do a lot of video editing and rendering, Quicksync and the T2 can speed up video rendering many times over! For example, a typical video may take 1hr for my Mac Pro with a RX580 to finish rendering to h.264 MP4, but it took only 30 minutes for my Macbook Air 2014 via Quicksync to complete without a RX580, but only with its built in Intel 5000 iGPU. But the Mini 2018 with Quicksync and T2 chip can finish it in about 10 minutes for what took the Mac Pro 1 hr or my Macbook Air 30 min to render in 1080p h.264. So if your thing is create lots of video for blogging and posting on Youtube, then skip the Mac Pro and go all in Mini. If your applications are balanced like what I do; Photography and video editing, then a Mac Pro is a better deal than a Core i3 Mini 2018/2020. If you can get a Mac Pro with the X5680/X5690 already installed in a dual tray, then the Core i7 Mini 2018/2020 would be a better choice.
Hope this helps.