Mine is a 2-car garage, some space is used by built-in cupboards, so there is 6.5m x 5.5m available to put gear. 0.5m at the bottom is clearance for the door, so in fact about 30m2. All my gear cost about $5,000.
In 3x4m you could have basically a power rack and platform with plates etc jammed along the sides. Bear in mind a barbell is 2.2m and you want a good space either side to load the thing, 0.4m would be a bit tight, and you'd have to lift across the 4m length of the thing for quick lifts, since when dropped the bar may go left or right.
With that space I would get,
12m2 of rubber matting @ $25/m2 = $300
Power rack with chinup bar 2.2x2.4m = $600
Power bar and WL bar = $600. If you're a woman or older person, get a 15kg bar as your WL bar so you can start there with presses.
mid-range 150kg bumper plate set = $800
Fractionals = $100
Moveable bench = $200
So that is $2,600 in all. The bar could be loaded anywhere from 15 to 180kg, which will be enough for most people for the first 12 months, or for less driven people, enough for a lifetime.
Technically you can do it all with a WL bar and bumpers in any 3x3m space, no rack needed, so long as you have a space to store the gear out of the weather. But then you have to learn cleans on day one which makes it more challenging.
Obviously you can look for deals etc, but this would all do for midrange new gear.