Perhaps a combination of the "strength" of the ability in the game and its frequency/activation:
- Little effect on the game = 5s
- Considerable effect on the game = 10s
- Dramatically affects the game = 15s
- Rarely useful (as in crits) = +0s
- Eventually useful (with 12+ on 1d20) = +5s
- Power (card destroyed if failed) = +0s
- Often useful (such as always active or passive) = +10s
So a considerable but rarely useful effect would cost 10s.
The keywords would probably also be worth some variable cost.
What do you think?