For a Family of four, the poverty level is $24,980 per year. A "prebate", they would get ($24,980 x 0.23)/12, or roughly $480 a month.
If you're looking at a senior couple, you're looking at a poverty level of $18,620. ($18,620 x 0.23)/12 ~ $357 a month in prebate.
The rebate of the sales tax on necessities means that higher consumption families would pay higher average tax rates. For example, because their first $24,980 was not taxed, a family of four spending $49,960 would pay an 11½ percent tax on their taxable purchases.
A family that spent four times the poverty level ($99,920) would pay an average tax rate of 17 1/2 percent.
That's over twice what they're paying today.
A couple earning $99,920 and spending all of it (a family of four surely would) would pay $17,486 in taxes (@ your 17.5%).
Today, that family of four would get to deduct mortgage payments (say $24,000), property tax (say $8,000), charity (say $2000), four exemptions (12,200), for a taxable income of $53,720.
Going to my 2003 tax tables, they owe $7359.
Bend over, family of four.