Ironic you write the vertical is more $; usually the other way around.
(Usually) already has a 4th axis
Shorter structural loop (Machine-Tool-Fixture-Work interface)
On an HMC you can machine 6 sides (or more) of a workpiece in 2 operations
Palletized machines enable change-over downtime on the order of 10 seconds
Large workpieces can be machined all the way around in one setup
Chips fall away from the work
More suited to unattended operation
Tombstones allow for multiple jobs to be set up at once or:
More operations for one job to be set up at one time decreasing lead time
Easier integration to other equipment using a palletized system (CMM, VTL, ?)
(Generally) better volumetric accuracy due to fewer axis stacked up
Size of parts have fewer limitations (doesn't have to fit under the spindle, can hang off the table, & the sky is the limit).