The best value performance parts are: Air Filters, Carburetors, Exhaust Manifolds, and Ignition parts (excluding the default "Ignition Coil"). For electric cars the Cables are a cheap boost too.

I found this out by creating a spreadsheet (Link)[docs.google.com] with all the tuning shop parts cost vs their performance gain.

It calculates each part's value divided by its percent increase in the "Cost per %" column, lower being better.

You can right click the top of column D and sort sheet A-Z to organize the sheet by value instead of alphabetically if you prefer.

I created this to find out which parts give the most performance for the least cash, so I could be thrifty while doing tuning jobs. Hope it helps!