The cheapest option would be less than $100. The better option would be something like this, for CPU upgrade.
PCPartPicker Part List:
https://pcpartpicker.com/list/z2MMQq
CPU: AMD Ryzen 3 3300X 3.8 GHz Quad-Core Processor ($119.99 @ B&H)
CPU Cooler: Deepcool GAMMAXX 400 74.34 CFM CPU Cooler ($29.28 @ Amazon)
Motherboard: MSI B450 TOMAHAWK MAX ATX AM4 Motherboard ($114.99 @ Best Buy)
Memory: G.Skill Ripjaws V 16 GB (2 x 8 GB) DDR4-3600 Memory ($82.99 @ Newegg)
Total: $347.25
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2020-05-22 10:49 EDT-0400
If better gaming is the goal, though, you might be better off with the Xeon upgrade, that I mentioned earlier, and getting a better graphics card. What is the make/model, of your power supply? That matters a lot, too.