The problem is diminishing returns. You can weld a jet engine to a sled, at the cost of a few grand and reach 500 mph without a hiccup. But what do you have to do to reach double that speed? You might need to make your sled aerodynamic. Alright, no pro- what about temperature? That's gonna be quite a lot of money, so add a few zeroes to the budget. But that's not all, your sled isn't going to remain in one piece for long and you need it to last long enough to reach 1000 mph. Ok, just rebuild it with strong enough materials, but also light enough so that you don't need to spend more on a better engine. But alas, you've added yet another zero to the budget but there's just no material on earth that's light enough to reach 1000 mph with the engine you have so that's even more spending on a new engine. Now do all of that while also adhering to the rules of "fastest ground vehicle with wheels". I wish it was as simple as you said but nowadays to break old records you need to push the limits and that gets expensive fast. Even ignoring safety and the ability to slow down once you reach your goal just doesn't give you enough freedom to do it cheaply.