Soil Tests are pretty cheap to have done and each Soil Test costs between $10.00 to $20.00 per soil test depending on what type of soil test you have done.
Some tests for farming to test for toxins can be as high as $50.00 per test but that's still pretty cheap.
If you want the test results of the soil test to be expedited then it can cost as high as $100.00
You should have the soil tested at least every 3 years.