Nothing used available? Can understand you don't want to upgrade a whole platform,but $300 is alot of money for just a cpu.
Already better,but because of ram more expensive,
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel - Core i3-8100 3.6 GHz Quad-Core Processor ($187.39 @ Amazon Australia)
Motherboard: ASRock - B360M Pro4 Micro ATX LGA1151 Motherboard ($109.00 @ Umart)
Memory: Patriot - Viper Elite 8 GB (2 x 4 GB) DDR4-2666 Memory ($78.10 @ Newegg Australia)
Total: $374.49
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2019-03-04 00:03 AEDT+1100
He could try and get some money back by selling the old parts.
Don't know what the gtx 1660 ti costs,maybe you might look at something else there? Can understand you want that though.