TYS is has never been cheap and things have worsened since the departure of Independence Air. 5-10 years ago I only dreamed of a MCO-TYS nonstop w/o ATL or CLT stops. Last year the market had up to 5 nonstops per day and Indy Air was selling them for $44 OW and DL for $59 OW. Now DL has the remaining 2 flts. per day and they run between $400-$1300 RT.

Before the nonstops, I used to find it easier to fly N/S to BNA on WN and rent a car for the 2.5 hour ride over to Knoxville. From Houston you may want to consider the same as cars are quite cheap at BNA at most times.