I'm going to place a bet on the idea that the article differentiates between a defined object and a concept. This doesn't discredit the other answers here which are respectable in their own right.
The concept of 'low probability' is just that: a concept, an idea. To attribute this concept to something like a procedure says that the procedure has a low chance of failing. 'A low probability' implies that there is a defined probability for this object instead of just declaring that there is a low chance of something happening.
For 'procedures with fixed cost' I would include the article, because in this example you would define a fixed cost: a 'fixed cost' isn't really a concept, nor is it the same for each procedure.
The same follows with the car example: each car has its own radio, hence 'cars with a radio', or, even better, 'cars with radios'.
The inclusion of the article implies a defined object instead of the concept of one. Consider the difference in meaning between 'drugs' and 'some drugs', or 'alcohol' and 'some alcohol' or 'friendship' and 'a friendship'.
Trivia: It's the opposite in Spanish. 'Las drogas' is the concept while 'drogas' is the objects.