Minimum of 20 seconds respawn delay after your dragon is killed. Then the cost to spawn it starts high and scales down the longer you wait to respawn it. EXAMPLE: If you wait 21 seconds it costs 100 followers. 30 seconds reduces it to 80 followers. 40 seconds reduces it further. Possibility: the more abilities it uses the more costly it is to respawn. Additional note: could use the level of abilities to determine cost.
Initial spawn would still be cheap. You could have dragon towers reduce the respawn penalties and/or initial spawn time
I like this, but I would add to that those follower numbers should scale with difficulty.
