NEW YORK (CNN/Money) – If the cap on wages subject to Social Security tax is raised or eliminated as part of a Social Security reform plan, sole proprietors of small businesses may feel the hit.

That is, if they are fortunate enough to make $90,000 or more in net income. (In 2003, 6.5 percent of unincorporated small-business owners did, according to Brian Headd, an economist with the Small Business Administration.)

Such sole proprietors may include anyone from business consultants to graphic designers to independent contractors and personal trainers.

Currently, the first $90,000 of a worker's wages is subject to a 12.4 percent Social Security tax. Employees pay half (6.2 percent), while employers pick up the other half.

But if you're self-employed, you pay the full 12.4 percent. You do get some of it back come tax time because you're allowed to deduct the employer portion on your federal tax return. But you won't get the full 6.2 percent back. You're more likely to recoup about a third of that, according to tax information publisher CCH, Inc.

President Bush has said repeatedly he would oppose any increase in the payroll tax rate (12.4 percent). But he has not ruled out raising the cap on the amount of wages subject to a Social Security tax.

Meanwhile, two more specific wage-cap proposals have been made.

Robert Pozen, the chairman of MFS Investments who has proposed that Social Security benefits be progressively indexed -- a move the president supports -- has suggested subjecting wages above $90,000 to a 2.9 percent surtax, half paid by employees and half paid by employers.

Pozen suggested that the employee portion of the tax could go into an IRA if the worker chose.

Rep. Robert Wexler (D-Fla.) has suggested imposing a 6 percent tax on all wage income above $90,000 – again, with employers picking up half the tab.

So what would that mean to a small business owner with a net income of $150,000?

Right now, sole proprietors pay 12.4 percent on the first $90,000 of their net income, for a maximum of $11,160.

If the Pozen plan went into effect, sole proprietors netting $150,000 would pay an additional $1,740.

Under the Wexler plan, they'd pay an extra $3,600.

Or let's say a reform plan passes where the cap on wages subject to Social Security tax were lifted to $150,000 and taxed at 12.4 percent. Sole proprietors with net income of $150,000 would pay an extra $7,440.

If the cap were only lifted to $130,000, the additional bite would be $4,960.

If wages over $90,0000 are made subject to a Social Security tax, that means small business owners will pay out more, but it also means they will have a lower adjusted gross income (AGI).

That's because an increase in their Social Security tax means they will be able to take a larger above-the-line deduction on their 1040s to reflect the employer portion of the tax that they paid.

"That could have some effect on your eligibility for other tax breaks," said Mark Luscomb, a tax expert at CCH. "In general, it would be a good thing for your AGI."

AGI is often key in determining whether you qualify for various tax credits and deductions. And a lower AGI typically means you're more likely to qualify for those breaks.

Some common ones, according to CCH, include the ability to claim:

• Itemized deductions -- eligibility begins to phase out at $142,700 for joint filers
• Personal exemptions -- eligibility begins to phase out at $214,050 for joint filers
• Medical expenses -- deductible only to the extent that they exceed 7.5 percent of AGI
• Child tax credit -- eligibility begins to phase out at $110,000 for joint filers
• Miscellaneous deductions -- deductible only to the extent that they exceed 2 percent of AGI
• Higher education expense credits -- eligibility begins to phase out between $87,000 and $107,000 of AGI
• Non-business casualty losses -- deductible only to the extent that they exceed $100 plus 10 percent of AGI