I think that is a valid point. However, I think 10% is a low estimate in my judgement, especially in the long run. But I would also like to point out the dynamics involved using your numbers as an example. Even if prices only come down 10%, a $10 item would be priced at $9. Adding 29.87% tax would yield a total cost of $11.69 including the tax.
If your estimate is low, and prices were reduced by 15%, the $8.50 item plus the 29.87% tax would yield $11.04.
Keep in mind they will also receive the credit to compensate for 23% of their purchases up to the poverty level. Also consider how the benefits are calculated. If the cost of living goes up, then the benefits should rise as well.
Is anyone considering the boon to business with all the corporate headquarters moving back to the US. With the US becoming a tax haven for the rich mucky-mucks... the economy will grow by leaps and bounds. Once that happens, the % of tax can be dropped.