This gets asked plenty around here…answer always the same, Threadripper or Core i9 with GTX1080Ti and 128GB RAM. The Xeon is likely not better than the i7 but I didn’t lookup the exact specs.
My relatively new Core i9 beast of a computer cost $6,000 if you want to explore all options in Pix4D and do it the fastest. Compromise down to fit your budget but you need 3 main things, GHz, Cuda cores, and RAM.
$500 won’t really get you close to the best possible Pix4D computer but you can improve your i7 with a new GPU.
https://community.pix4d.com/t/4079-Pix4D-Computer-hardware-community-benchmark#gsc.tab=0