I don’t know…at least not yet.
Roughly I get at least 10-15 flasks each month from challenge events and drops which I save for AR. I am above level 56, so thats roughly 450-675 WE at least. I’m nearly F2P because I stopped buying VIP, so I don’t have stockpile of tickets either and not interested in paying for those either. Mostly auto farm lower levels and I think I usually am able to scrounge enough coins for at least one summon each month. Not gonna lie, this is a LOT of work and may not even classify as free depending on how you value your time. In contrast costume and ToL takes a lot less time and have different pros/cons.
This thread has better quality data shared and might be a good starting point - How rare exactly is the Atlantis Rises seadragon? (10% chance)
I’ll try to measure how much WE I expended and how many coins I collect this time and share on that thread.