Greatly increasing the plot prices doesn't turn housing into a true gil sink, though.
True gil sinks affect a majority of players, slowly draining gil that's in active circulation from the game economy to offset the new gil being generated through game play. The amount being drained doesn't unduly burden less wealthy players or block them from content.
Increase housing prices to an extreme level and you're not draining the gil in active circulation from many players. You're draining inactive gil from a few wealthy players that had been hoarding it, gil that likely would never have made it back into active circulation.
I can think of 2 methods for housing to have effective gil sinks. The first is to move all housing items currently acquired through crafting/loot drops to vendors and sell them for gil only. It's still not a great gil sink because it's dependent on how much a player wants to decorate/redecorate but it doesn't unduly burden less wealthy players since they can choose how much they spend.
The second has been suggested before and gets a lot of hate. Add a monthly property tax. Set the value to a few percent of a house's minimum purchase price, say 2% for smalls, 3% for mediums, 4% for larges. Taxes on the smalls would be affordable for less wealthy players. Taxes on the mediums and larges would still be reasonable for wealthy players who make more gil. FCs running an active workshop shouldn't have a problem paying the tax on any size house.
Such a tax would steady drain gil from a larger portion of the player base like a good gil sink should do.
It has the added benefit of reducing demand for houses, leaving more available for those who would actually be using them. People are a lot more likely to make purchases that have a one time cost than they are to make purchases that involve a recurring fee.
How many players not currently using their house would keep it if they had to pay a monthly tax? How many would demand more medium and large houses if they had to pay 10-40 times more a month in taxes than they would have to pay for a small?
If SE is going to stick to the ward system, a tax would do a far better job of leveling out demand than increasing the one time cost would. Players could go the RMT route to buy what they want if they needed to. They're not likely to go the RMT route to pay a tax since their normal game play (whether running roulettes or selling things on the MB from crafting/treasure maps) would already provide what they need.