I didn't make clear in the first paragraph, but ultrasounds are screening tests, not diagnostic tests. The expensive blood tests are diagnostic. Screening test

I didn't make clear in the first paragraph, but ultrasounds are screening tests, not diagnostic tests. The expensive blood tests are diagnostic. Screening tests are inexpensive, low risk, high recall (i.e. high true positive) tests meant to indicate if high precision (i.e. low false positive, low false negative) test is warranted. Often the diagnostic tests are more invasive, potentially riskier, and more expensive.