The difference is in the re-usability factor. The number on the Falcon 9 cost page is as follows:
- Falcon 9 to GTO- 5.5 t to GTO \$62m
- Falcon Heavy to GTO- 8 t to GTO \$90m
However, it lists the capacities as follows:
- Falcon 9- 8.3 t
- Falcon Heavy- 22 t
One can only assume that the reduction in price is for a mission that allows for landing the booster again, to try for re-usability. The full capacity, that doesn't allow for landing, costs more. There might even be a likelihood of return computed, a booster that needs to carry more weight has a higher chance of being damaged.