Well, so far my two most expensive purchases were a Nikon FM2n kit (24, 50, 105) for about 600$ and a Mamiya C330 TLR (105mm) for about the same price. Of course, I can add more bits and pieces to the kit over time, but in terms of onetime expenses, I would set my limit between 1,000$ and 1,5000$.
That would still be enough to get me a nice Mamiya 6 kit, or maybe a bargain Leica kit, a basic Hasselblad kit, or a Rolleiflex 3.5F in good condition. Which for now is as much as I should care to desire (students loan anyone?).