I expect higher prices on Modern Horizons in general.

Ultimate Masters is fundamentally different situation... stores didn't even know of it's existence until November 5th for a December 7 release.

Ultimate Masters was a limited product, and it was released at the start of December. Because most game stores follow a Jan 1-Dec 31 fiscal year, they wanted to be sure they're not holding a lot of unsold boxes of $220+ product... given the 12-7 release date, this meant from the point where they were able to sell it, they had 3 weeks to blow this out the door or have it show up in their unsold inventory.

They were very much incentivized to turn and burn this stuff, happily taking the $30ish a box.

Modern Horizons will be an unlimited print run and be released in June.

That means no one is really under any pressure to kick this stuff out the door with the sort of haste you saw with Ultimate Masters.

While I do expect you will have stores that are happy to undercut and deal in volume as we have all the time, I don't expect all that many average game stores being willing to bite on a particularly thin margin. Especially if War is successful.