I think this is going to be way more expensive than you imagine. Let me try to break it down:
Your Goal - Add flight as a second trait to your civilization
Payment - 10 labor + XFP from various gods
In this situation what you are contributing to the goal is meaningless, 10 labor is what it cost Jotun to build an Orchard. It would cost way more than that to add a trait just for game balance reasons.
So the only way this is possible is basically entirely depending on the gods, and that would likely take at least 20+ combined FP I'm guessing. If I were running the game I would actually make it something like 50FP and then something like 200FP for a third trait.
Edit: That's from the perspective of a god using FP of course. I guess since I've been so negative I'll try to be more constructive and give some suggestions from your perspective.
From a civ's perspective trying to develop a second trait should probably be thought of as a 'wonder'. That is first you should research tech to actually accomplish what the wonder does, then tech to contruct it, and finally put in tons of labor to actually build it.
Example: The Grand Dam - A massive dam that controls a river, it prevents droughts and flooding and boosts agriculture along the river.
First research Irrigation, Flood control, and maybe Aqueducts and Wells - This lets you actually do what the wonder is for, controlling water flow
Second research all the construction techniques needed to actually build it, an example might be Ancient Concrete (like the Romans used) to create a watertight seal
Third actually gather the materials and spend tons of labor building the damn thing
You currently don't know how to fly with magic at all, let alone innately, you should probably research spells that let your mages fly as a baseline. Then research tech like Enchantment and Ritual Casting so that you can actually enhance your population. Finally train a bunch of mages and have them cast a massive ritual to enhance your people, likely taking a lot of labor and many turns.
These are all rough examples because nobody has actually built a wonder yet and we don't know what all the GM will decide is necessary for them, but this is just my suggestion on how you should be thinking of this.