Assumptions, model and detailed results given below. If you want the Excel spreadsheet to play with on your own, leave a comment below and I'll send you a copy.
The following graphs express the cost of vouchers as a percentage of annual income assuming that half of eligible children attend private school in a 2 parent home (except for single parent and single child homes). In the graph, the horizontal axis is annual family income. The vertical axis is number of children attending private school. The values on the vertical axis are 1, 1, 2, 3 (sorry for the lack of labels, my medicine kicked in and it happens to cause excruciating headaches and fatigue).
I decided that "affordable" means that tuition accounts for 5% or less of the family income. Affordable is colored blue. "Might be affordable" means that tuition is between 5 and 10% of the family income. Scenarios which might be affordable are colored orange. "Not affordable" means tuition is greater than 10% of the family income.
First up, we'll assume private school costs $4,500 per kid as assumed by Parents for Choice in Education.
Without vouchers, private school is affordable for only families making more than $110,000 and with less than 2.5 kids in private school. Not so good. Private school is maybe affordable for families making more than $60,000 a year and for larger families with more income.
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The impact of vouchers on affordability with tuition at $4,500 is striking. With vouchers, all but large families with low income can afford or might be able to afford private school. Using the PCE tuition assumption, vouchers have a significant positive impact on affordability.
Next, we'll redo the numbers using the amended Deseret News numbers, which includes some kind of weighted average tuition which is $6,935.
If tuition is $6,935 then private school is affordable without vouchers only for small families making around $150,000 a year and might be affordable for small families making $70,000 or more.
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Vouchers have little impact if tuition is $6,935. There's no significant change in families that can afford private school. Some small families making around $50,000 may be able to afford private school.
We happen to send our twins to a private preschool, all preschools are private so no sympathy cards are being played here, and I sanity-checked my affordability assumptions on our family budget. No problem. We'll send them to private kindergarten next year and kindergarten will be affordable because we'll make it affordable, but that's another story for another time ... maybe later this week depending on how things go.