4 branch manifold, ported head, ported throttle body
to make it rev faster it needs to be able to take air in at a greater rate with least resistance and to do the same getting it out
= ££
Its only expensive if you dont do it yourself, porting can be done fairly easily.
4 branch is a bit more difficult, but something to save up for after you have done all the free/cheap bits.
I would like to add, most modifying costs are relative to how handy you are.
On N/A engines you can achive a fair bit on a low budget if you have the time and dedication in doing so. in that lies the reason it is easier to stick a turbo lump in, more power at more cost but less time in the garage

My 2P anyhoo