If cheap, abundant, shipping containers pull 25% of the demand out of the low-end multi-family segment, that's a 50% drop in prices. Low-end housing is a substitute good with mid-end housing for at least some of the demand.
The prime example are people who live with roommates in mid-end housing, who with cheap enough low-end prices, would get their own place. A 50% price shock to low-end multi-family at the very least would pull 10% of the demand out of the mid-end. That would lead to a 20% drop in mid-end multi-family prices.
Just the same there's at least some substitutability between mid-end multi-family and mid-to-high-end single family. A prime example are empty-nesters looking to downsize. With a 20% price shock to multi-family, that would suck at least 5% of the demand out of the single-family segment. That's a minimum of a 10% price reduction.
Hence a positive supply shock in the form of cheap, abundant shipping container homes would reduce the price of mid-high-end single family homes by at least 10%. This is all micro-econ 101.