No beginner ever works up a sweat dancing a waltz . . . .
The accuracy of this statement depends entirely on the teaching method.
If the beginners are working with a poor instructor who spends most of the lesson time ONLY on step patterns then this observation is completely correct.
Beginners are most likely to encounter this brand of instruction at the chain schools. There are more reasons to avoid them than expense although that reason is certainly the most practical.
However, if the beginners are lucky enough to get an old school instructor who drills them in footwork, legwork, posture, and body control (CBM, etc.) along with the patterns then they'll work up a sweat much sooner.
The body works hardest when it's learning new and unfamiliar movements. Once mastery is gained and the body becomes more efficient, the one-time beginners will realize their benefits from being able to dance continuously for longer periods.
I personally don't think that DWTS has been good for ballroom.
Good luck finding a studio. I'd start on the 'net by checking out lists maintained by various organizations such as USA Dance, which used to be an amateur dance body.
jj