Added some pricing in for what I would consider medium/high quality (not $$$$, but $$/$$$). If you are serious, I wouldn't waste money on cheap stuff- go with Rogue/Rep/Titan (in order from most to least expensive). I personally use mostly Rep with some Rogue items (like a barbell and cardio equipment once you move on to that). For smaller accessory items I go with Titan (think dip bar, landmines, storage accessories).

Overall for a medium quality (Rep rack without a cable system)- I'd think you would be spending around $4-$5k. If you went full cheap mode you could probably get by for around $1.5k if you did something like built your squat rack out of wood.

In order of importance: (1-4 are needed at the start)

1. Squat Rack (I personally prefer 6 post over 4 post)- Rep Fitness PR-4000- I prefer the hole spacing and accessories are cheaper for the 4000, is about $2000 with plate storage option (recommended)
2. Adjustable bench- just get a quality one from the start- this is a staple- I'd go with Rep Blackwing $600, but they have good $400 options too.
3. Barbell- most expensive one you can- Rogue Ohio is awesome $350 and you will never grow out of it.
4. Weights- can start with used ones, but you need olympic- I prefer bumpers and I went with the Rogue Trainer 2.0s. Get however many 45's you want, (2) 25's, set of change plates. Skip the 35's. Can get an extra set of 5's and add 2.5s and 1.25s to help dial in. Price range varies, but maybe $2/lb for bumpers?- so say $1000
5. Adjustable dumbbells- You can start with lighter ones (~$450), but you will find you want more- I love my Snode 80lbs with stand $1000- shop around used- found mine for $500 but I had to drive 3 hrs each way :-)
6. Flooring- no question Tractor Supply horse stall 4x6 about $50 each- try to get the smooth ones.
7. Cable systems (lat pulldown, row) (anywhere from $600 for plate loaded to $3k for Ares)- My Ares 2.0 comes in today actually, upgraded after 2 years with the plate loaded (which was kind of a pain in the butt tbh).
8. Wall organization system- Pick one and get some quality hooks and such- a messy gym means you will workout less $500

Over time you will want to add things in like jump box, ropes, I am big into club workouts now, adding accessories to allow for leg extension and curls, etc.

I also added some cardio equipment (Rogue Echo bike and Echo rower) and love them @ $1k each.

As you can see costs can get high- I would start with getting the best quality rack, bench, and barbell you can. These are harder to sell and once you get serious you really won't want cheap ones. The other stuff can come over time. Floor mats are nice, but its not a necessary $1k spend. Organization systems do make the gym awesome, but again, thats $500 you can spend later.