Book value on uncirculated 1968 20c is $65 (Though a more realistic price to sell/buy would be around the 20-25 mark. It is second only to the 1971 which comes in at $70.
Obviously varieties are worth more; 1966 Wavy 20c and 1981 3 1/2 claw.
The mintage of the 1968 was quite high compared to years around it and is comparable to some recent years! However, the RAM made did not produce any uncirculated sets for 1967 or 1968 and this means there are fewer uncirculated coins floating around and I think that contributes to their price premium compared to mintage.
Hope it helps.