According to Ralph Mowery :
While a watt is a watt with lights, the 12 volt system will cost about 20%
more to run if the same wattage of bulbs is used. This is because of the
transformer loss and a small aditional loss in the 12 volt wiring compaired
to the 120 volt wiring..

True, however, outdoor LV systems are generally not operated
with the same intent as indoor. You're not trying to achieve
the same lighting level, they're often just markers or just
intended to be enough light to avoid tripping over things.
As a result, most systems don't really use that much wattage.

A lot of installations are just short strings of 4W bulbs.

The 12 volt system is used mainly due to the safety factor.

And simplicity of installation.

As far as "economy" goes - halogens will give you slightly more lumens
per watt, but not likely to be useful in reducing power. It's better to
be more modest in lighting requirement and simply reduce wattage or the
number of bulbs. Or, if you have money to spare, go with LED bulbs.
_Vastly_ more efficient than even fluorescents. There's some really neat
new bulbs becoming available. Eg: 50W-equivalent LED bulbs in GU-10 or MR16
bases that draw only 2W of power, and undoubtably some that would fit
just fine into traditional pea-bulb LV lighting systems like Malibus.
Unfortunately, the prices are still heart-stopping...

[IIRC, 12V 50W-equivalent MR16 LED floods drawing about 2W of power
are over $50 _each_. Theoretically very long lived however - 50,000
hours and up. As long as nothing bad happened, it probably still would
be a good investment in terms of power savings. But the startup
costs are enormous.]
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.