The "fallacy"of the *sunk cost fallacy" is people getting emotionally attached to money that has already been spent / time that's already been wasted / other resources that have already been used, and despite knowing or having good reason to believe that future resource use is not a good investment given the current state of affairs, continue using resources.

It is not, however, fallacious to observe that money has already been spent to make some kind of progress, that spending only a little bit more money will actually get you a useful product at the end, and that abandoning the project entirely will get you nothing of value. You are making a rational assessment to continue investing money because, with the state of affairs as it is, additional investment appears to be profitable.

Let's say, for example, that you signed a contract to buy a new Ford F-150 for a million dollars, paid in $1,000 installments, with the vehicle to only be delivered if and when the final payment is made. Otherwise you get nothing.

That would have been a stupid contract to sign. It would be stupid to keep paying on that contract if you had only already paid in $1,000, or $10,000, or $100,000, or $900,000. The value of the vehicle is not a million dollars. It's $100,000 or less. Every single payment you make up to roughly the $900,000 level is objectively a bad decision, even if you've already paid in a substantial amount of money. However, along the way, your decision to keep paying might have been driven by the sunk cost fallacy. After all, you already threw $100,000 down a hole. If you stopped paying now, that money would just disappear to no benefit.

On the other hand, if somehow you inherited the right to be sold the F-150 knowing that only a single $1,000 payment needed to be made to actually get the car, it would not be fallacious reasoning to make that payment. It wouldn't make it fallacious if you observed that the $999,000 already spent are a sunk cost. You would be making a rational decision to spend $1,000 in return for an F-150. Somebody else might have made a bad decision to pay up to that point, but it's not a bad decision to pay just a little bit more money to get a useful product at the end.