I recommend [URL=]http://eh.net/hmit/compare/ (linked off of one of the pages recommended by crowmanyclouds), because it gives estimates calculated by several formulae and a little help interpreting each.
The fundamental problem is that the shape of the economy has changed so much since the 1850’s that there is no single conversion factor: the rich are much richer, but the poor are much less poor and the middle class is much, much larger; food is much cheaper but labor is more expensive; and things that we now consider the minimum required for survival (indoor plumbing, post-elementary education, transport other than by walking) were luxuries restricted to the relatively wealthy.
For example, almost every family now has at least one car, even the very poor. The 1851 equivalent would be a horse and carriage; unless you had a farm and could grow your own hay and grain, supporting a horse was an expense that could strain even a comfortably middle-class income.
So here’s there results for $500 in 1851 in 2004 dollars (sorry, not 2006 dollars, so add in a fraction of a percent):
$12,278.48, using the Consumer Price Index, which compares the cost of a “basic package of goods”. The ingredients of this package correspond to a lifestyle that would be considered low-end working class today but fairly affluent in 1851; e.g., it assumes a car or equivalent, a varied diet, medical care, education expenses for children, etc.
$92,659.31, using the average unskilled wage. This is a pretty useful index for earlier time points, since a large proportion of the population in 1851 was unskilled labor. On the other hand, the difference in income between unskilled labor and middle-class was larger then than it is now (as you can see from the difference between these two values).
And finally a few indexes that consider the average value of the entire economy (see website for details):
$9,586.99, using the GDP deflator.
$178,234.06, using the nominal GDP per capita.
$2,173,018.52, using the relative share of GDP.
So, in summary, $500 in 1851 would have been about 4-6 months operating expenses for a moderately affluent household, or a small fortune to a poor household.