Probably a MiG-21. Some flyable examples are available for between \$50,000-\$200,000, though I suspect some those airframes in an excellent condition will run upwards of \$1 million.
Operational costs for MiGs tends to be surprisingly cheap, though considerably more expensive than operating exotic sports cars or other millionaire toys. At an estimate, I would guess $3000/flight hour for a MiG-21 and it all depends on what equipment is aboard eg hot ejection seats, etc.
Other popular types are L-39s, Folland Gnats, A-4s, F-5s, T-38s, even a Panavia Tornado and a Harrier on the market. They can run from $200000 to tens of millions of dollars and cost thousands of dollars an hour to operate, depending on type.
But aside from the purchase price, if you’re a newbie to flying, it’s gonna be a while until you are legally capable of logging PIC time in one of these jets, at least in the US. The FAA has specific guidelines on piloting these kinds of Experimental category surplus military jets, which have to be followed. See AC 91-68. You’ll need at least 1200 hours TT plus at least an existing PPL rated for the category and class of aircraft, then undergo further training and pass a practical test to get an LOA for PIC operation of these kinds of aircraft. Type ratings also apply, if the airplane in question meets the characteristics listed in §61.31(a).
I can’t speak to Peter Kampf’s comment; other countries may allow you to fly such an aircraft without additional training or aeronautical experience requirements but keep in mind that going into this kind of an aircraft, it is a hot blooded machine that demands to be flown by the numbers and will not suffer a fool or a neophyte at the controls. While it is true that military forces throughout the world allow their pilots to operate these airplanes at far lower time levels, in many cases they pay for it through the nose with a high accident rate. The USAF alone in 2018 had 716 mishaps with 19 fatalities during flight operations. More that 15,000 US military airmen were killed in flight training accidents during WWII.