Like the income tax there is not really a formula to compute the tax; but rather it is based on tax tables after allowable deductions (or exclusions).
For the gift tax the first $12,000 (in 2007 and 2008) is excludable and no tax is due and a return is not required unless the gift is more than $12,000. See this article for more information.
When one half interest in a house is given, the value of the gift will be one half of the fair market value of the house on the date of the gift. Either an appraisal or a comparable list from a realtor could be documentation of the value of the date of the gift.
For example, if the house if worth $300K at the date of the gift there would be a gift of $150K to the child for the one half interest. On Form 709 the taxable gift would be 150K-12K = 138K. But, there is no gift tax due when filing the return unless the donor has already used the lifetime exclusion amount of $1,000,000 unless an election is made to pay the gift tax. So, except in a few instances the taxable gift of 138K does not result in any current tax; but the 138K will reduce the amount that can be excluded from the estate of the donor when estate tax is filed. See Publication 950
The rates for gift taxes and estate taxes are generally the same. There is no federal inheritance tax.
If you believe that the parent will incur estate tax when they pass you are well advised to discuss the advisability of gifting now (or paying the gift tax now) rather than inheriting and paying estate tax later.
I hope this helps for the basic concept of how gift tax is computed even though there is not a formula.