score:4
Without a clue as to where you are in the vast Houston area, the least expensive is by bus, the 102 Bush IAH Express which runs from Travis Street at Texas Avenue to Terminal C. The cost would be less than $2.00
The very cheapest would be the free airport shuttles offered by hotels and motels in the area but, of course, you would have to have been a guest.
Shuttle and Uber will run $20-$40, again, depending on start point. Rome2Rio can help you plan the ride.
All are reliable, but are affected by traffic conditions.