There are several levels of testing.
- general testing - generally run in half hour to 45 minute sessions alternating between closed wheel and open wheel single seaters, usually the day before a race meeting, occasionally mid week too.
- semi-exclusive testing - run in an open pitlane format, with a morning session and an afternoon session. Generally only 12-15 cars at the circuit, whom are paying a premium for extra track time.
- exclusive testing - generally hired for one series only rather than the disparate mixture of cars you get in general / semi-exclusive testing.
- private testing - if you're wealthy you can hire the whole circuit. Whether you allow the public in or not is your call.
Generally testing is free to get in, although not always - Oulton Park were charging £ 5/head on Good Friday for British GT meeting testing.