Image: Tax forms © C Sherburne, Photolink, Photodisc Red, Getty Images

Taxpayers are always on the lookout for tax deductions, tax credits and income exclusions that help trim what they owe the Internal Revenue Service. What we call tax breaks are known as tax expenditures on Capitol Hill. And they cost the U.S. Treasury a lot of money.

Earlier this year, in preparation for a Senate hearing, the Joint Committee on Taxation calculated how much money is lost to some popular tax breaks. The final math? The top individual tax breaks will cost more than $3 trillion in uncollected taxes between 2010 and 2014.

The new committee charged with finding ways to trim the U.S. deficit will focus on spending cuts. But it's a good bet that some of the panel's 12 members also will look at how much money the Treasury could collect if at least some tax breaks were eliminated or tweaked.

Keep reading to find out how much Uncle Sam is expected to lose under the current tax system on your favorite tax breaks.

1. Health insurance

All employee compensation is subject to tax unless the tax code specifically excludes it. That's the case for certain employer-provided benefits.

The most tax-costly company perk is health care. The value of what your employer pays for worker medical insurance premiums, long-term-care coverage and health care doesn't cost you a penny in taxes.

But this break does cost Uncle Sam. Through 2014, employer-provided health care benefits will keep the U.S. Treasury from getting its hands on $659 billion.

2. Mortgage interest

One of the biggest individual tax breaks allows homeowners to deduct the interest they pay on their mortgages. It's claimed most frequently on the loan used to buy a taxpayer's main residence. But the mortgage interest deduction also can be claimed for second homes.

And those multiple residences don't even have to be permanent structures; a boat or RV could count. Supporters of this tax break say it's integral to making homeownership possible and keeping the housing industry afloat.

But it also comes with a high cost to the Treasury: $484 billion in lost taxes.

3. Capital gains and dividends

Investment earnings get preferential tax treatment and historically low tax rates for capital gains and dividends -- 15% for most taxpayers, 0% for some. These rates are scheduled to continue through 2012. The argument for the favorable tax treatment is that it encourages people to save money and invest in stocks, which keeps capital flowing into the economy and provides retirement cushions (that is, if the market doesn't totally tank).

But the cost of low investment taxes to the U.S. Treasury comes in two forms.

Investor savings, thanks to the lower tax rates on profits when they sell, are projected to reach nearly $403 billion by 2014. That gain for investors is Uncle Sam's loss.

Then there are assets left when their owners die. The increase in value of those holdings isn't taxed when the owner dies. That's because any heirs who get the property can step up the asset's basis, reducing any profit on subsequent sales. That produces a smaller tax bill for them. The cost to Uncle Sam, however, is estimated at $194 billion.

4. Pension plans

Another popular workplace benefit is a retirement plan. As with employer-provided health care, the uncollected tax costs are large.

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Defined-benefit plans, usually referred to as traditional pension plans, pay retirees a fixed amount based on salary history and length of employment. Employers make tax-deductible contributions, and, as plan earnings accumulate, they are deferred from income tax. And even though workers will owe taxes when the retirement income is received, the tax cost of this type of plan is estimated to reach $303 billion between 2010 and 2014.

Many companies have switched to defined-contribution retirement plans. Here a worker's future retirement money depends primarily on the worker's own contributions, though some businesses match at least part of the employee contributions. The most common type of defined-contribution retirement plan is a 401k, in which taxes on the contributions and earnings are tax deferred until the worker takes out the money. These plans are estimated to cost the Treasury $212 billion.