This comes up often and yet no matter how much we try to explain what it's like to grow them, people still complain about the price.
So out of curiosity, what do you expect the process to grow Thavnairian Onions is, and how much do you think the out of pocket costs will be to do it if you try to shortcut some of the steps?
Current lowest prices on the Crystal Data Center:
Thavnairian Onion - 138k (Brynhildr)
Thavnairian Onion seed - 125k (Goblin and Malboro)
Those aren't crack prices. The crack prices were back when BLU first came out and everyone was using BLU to level up their chocobos fast so they were going for 750k-1 million each. Do a typical 2 hour map party and you'll walk away with enough gil to buy 2-4 onions at the current normal market prices depending on dungeon RNG.
As someone who's grown the onions for personal/FC use on occasion in the past, I highly recommend just buying them off the MB. They're really not worth growing unless you're going to do it long term for profit. It doesn't that take long to earn the gil and you don't need them that frequently unless you're going out of your way to hard grind your chocobo XP.
Also, since you're saying you just hit max rank and can't go farther without the onions, are you aware you can get 2 as quest rewards? The first quest is Landing a Stable Job in Ishgard (the Jeweled Crozier area) (quick and easy, takes less than 5 minutes). The second quest is A Hunter's True Nature in Tailfeather, which is the middle of a side story quest chain that starts with I Wanna Be the Hunter (get from Loupard). Expect an hour to get all the involved quests completed.
What FCs that can't get a house?
Every world has available FC houses at the moment. Even after the ward redistribution on the 10th, FCs won't be in danger of not being able to get a house. If the available houses in wards 1-9 disappear, SE will move some of the other wards back to FC purchase.
If your FC is having a problem winning a lottery at the moment, make certain to wait until the last possible moment to enter and pick a plot with no other entries.