We were lucky in that NBN FTTP was required to be in area before build by council, and we got our sparky to lay network cabling at same time as electrical, so even if I could give you a rough cost it would be way out lol.
I did a lot of research first though as NBN was not due in our street in Sydney for another 4-5 years when we left - move from major city to bush for faster internet, seems legit lol. Cowboys were mentioned above and you can find them even when only in the research stage - had one company say that to properly future-proof our home we should lay fibre optic at build stage since we had FTTP. I laughed and reminded them of which country we lived in and the type of government leadership we can expect/have received when it comes to good technology outcomes - so fibre-optic cabling in Australian homes to future proof won’t be needed for decades.
I was leaning towards Cat 5e for a while but with my wife still working for Sydney Children’s Hospital, myself doing a lot of online stuff and a growing young boy and his visiting teenage cousins we went with Cat 6. I chose this because we have a single story home that is on the large size (acreage style home) and cable lengths to some areas would be long. On longer cable lengths Cat 5e can suffer under high bandwidth load, but Cat 6 can handle it. Would have gone with Cat 6a but at the time there was a noticeably higher cost and would only be for future proofing as the majority of domestic routers would bottleneck the higher speed anyway - same goes for Cat 7, Cat 7a & Cat 8 when they come out. The extra shielding in Cat 6 and above was a factor too, helping to minimise crosstalk & noise interference.
Once you lay your cables (whether at build time or down the track) you’ve effectively done some future proofing anyway, as it’s easier to lay new cabling where there is pre-existing cabling in most cases. Especially if you get your sparky/cabler to install with conduit in wall cavities, etc.
Quick word on wireless - good for many things but too unreliable for some things. Watching videos, checking your emails or even playing online games on your Ipad or Notebook is fine if you are close to the router, but scratch online games and hi-res videos if others are on the same network also doing high bandwidth stuff (depending on your router & network setup). For MMO games drop-outs can be a major hassle and wireless does not provide a constant connection, as Citrix is very fond of telling my wife when she connects through wireless lol. If you are not gaming or using software that requires the connection to be constant however, wireless is fine for now. The reasons stated here are also why I hate the idea of wireless NBN installations, just call it wireless not broadband (as broadband is used these days). If you are looking for future proofing and adding future value to your property then lay cabling as soon as you can afford it. The price of doing so will only increase as each year goes by even as the technology improves and becomes cheaper - tradie rates have never tracked downward lol.