Waldorf schools come in a pretty board range of costs (some run themselves on a pledge system for instance). Tuition amount may depend on things like the school's age (newer schools cost less), depth of program offerings, relative adequacy of employee salary and benefits, location, local demographics, and amount of administrative and remedial services offered.
$12,000-15,000 is definitely on the high side for waldorf schools, but I am sure you can find private schools that charge much more. My school will charge 9400 next year (we're in the northeast, but not around any major city).
As for how people afford it, some get help from their families, some apply for need-based assistance that most schools offer to differing levels, some reorient their finances to allow tuition payment. Some can never afford it.