if you wanted a exact order, you could
1st - all dragons.
2nd - the nightfalls.
3rd - from here on, it doesn't matters. you should start with dusk mantle so it costs less.
4th - I think, maybe, the steals. to cost less
5th - liquid shadow.
6th - obsidian pillars.
ps : if you change anything for a devourer, the devourer comes 2nd in upped. so they are more resistant against critter attacks. or if you don't want to up the devourers (if you even use them at all) change your mark to earth so you can burrow them. ^^