I have to express that it was an incredibly poor decision to add walls to retainers.
4.1 hasn't been out that long and someone in my FC has already brought back 3 different walls, including the exact eatery mansion walls I spent hours gathering and crafting for. Yes, mansion walls, lots of materials to make and limited in the number of mansions as it is. The price is already down to about 200k.
In the future, SE, you should seriously reconsider adding something so difficult to make to something designed to be simple as a "send it out, come back tomorrow and with a single click collect some easy rewards" content.
Considering how often those types of walls sell to begin with the market will be flooded at this point indefinitely, even if the current rate of procurement were reduced. And I would say in the very least the amount of materials needed to make such walls should be cut drastically. There is no point in even doing those projects anymore, as even at several million on the MB (extremely generous), it's still cheaper to buy than to make.