There are enormous variations in pricing depending on the region, the specific market, the needs of the particular planter and seller, the features of the slave in question (age, sex, physical capacity for labour, history of obedience, lightness of skin [light-skinned women especially tend to fetch higher prices], skill training etc.) and so on and so forth. Broadly speaking, around the late 1850s you're looking at an average price of around $800 - $900 per slave. This is an average that does not discriminate on the basis of age, sex or physical ability; it thus includes children, the infirm and disabled, the rebellious and so on.
To get an idea for the scale of variation and the kind of characteristics that would influence slave prices, take a look at this extract from an abolitionist's account of a slave auction in Montgomery, Alabama held on March 24th, 1854:
Woman and small child, $1050; man aged 19, $950; man and wife aged 18 and 17, $2000; boy aged 14, $640; girl aged 10, $525: man aged 24, $860; boy aged 11, white, $585; boy aged 11, white, $625; woman aged 25, $900; man and little boy, aged 50, $1020; woman age 46, $395; man, with the gravel , aged 19, $700; man, perfect aged 40, $1600; woman 40, girl 8, $600; man aged 27, $1410; boy aged 12, $725; girl aged 4,$300; girl good looking, aged 14, $855; girl, a little blacker, aged 15, $845. [from The Liberator, "From the Car Leader", April 21, 1854]
There are 23 slaves listed here, fetching an average price of $721 each (some are sold together); the average for an adult male slave, however, is closer to $1,000; adult women just $624, and a woman in her 40s is worth more or less the same as a little girl aged 4. Slaves in this particular auction sold for as little as $300 (a 4 year old girl and a 40 year old woman) to as much as $1,600 (the "perfect aged" 40 year old man).
In terms of what these values are in money, adjusting for inflation, that's the equivalent to about $23,500 in 2014 dollars; for reference, the median per person wage at the end of 2012 was $26,695 (and in 2012 dollars, the slave price is $22,800). In other words, we can say that the cost of a slave in 1860 was - in terms of the value of the slave as a commodity - roughly equivalent to one year's salary for the average American worker in 2012.
But money is more complicated than that. Inflation isn't the only economic force to account for in considering how much $800 in 1860 is worth today; even adjusting for inflation, put simply, the US is a much wealthier country (in a much wealthier world) than it was in 1860. There is more money to go around. As a share of the total value of the economy, $800 is the equivalent to about $3million USD today. So $800 in 1860 can only buy you what $23,500 can today - but in a less affluent economy, actually having $800 in 1860 essentially means you would be as wealthy (wealth being relative, a measure of power, rather than an absolute) as someone who has $3million today. Samuel Williamson and Louis Cain have a fantastic, publicly accessible article about how we measure the value of slaves here.
So as you can see, a slave was a substantial investment, and slave prices were extremely high on the eve of secession. Context matters though; whilst we can speak of an 'average slave' price, such a measure is perhaps not that useful for historians due to the significant variation from context to context.