i already explained my reasoning above; it's obviously correct, and it doesn't depend on my knowledge about which small tumors will become life-threatening, so it sounds like you didn't understand it
you can calculate the cost and expected morbidity due to any particular candidate test (mri, blood panel, biopsy, ct, pet, whatever) and compare it against the reduced expected morbidity due to the probably-not-present disease, using whatever metric of morbidity you like best (ypll, daly, expected change in your malpractice insurance premiums, whatever), using whatever risk tolerance you like best
for mris and blood panels obviously this will favor doing the test in almost all cases, unless traveling to the hospital is a significant hardship or the money is enough to be important to you
for other tests it may not, and watchful waiting is often a better choice than doing further tests
with a little work, you can always beat the best research hospitals at this task because you care about different things than they do. this is not the same thing as beating them at guessing whether your prostate is going to kill you before something else does