According to this paper (page 11 of the pdf) a reasonable estimate for the annual upkeep fees of a slave would be $20-$21 in the years 1840-60. Plugging this into a purchasing power calculator gives us a range of $556 to $607 in 2013 dollars.

These kinds of "what would x cost in today's dollars" comparisons are incredibly difficult and usually kind of pointless. The best we could do is come up with some kind of "average" price for slaves, the costs of feeding/caring for them, their productivity, etc. But each of those "average" figures would hide a tremendous amount of variation, and if you narrowed the question down to certain regions, types of slave, types of industry/production, then you would get answers that are totally different. And even if you could do all of that accurately - which is a serious challenge - then the conversion to modern USD is really little more than a guess. There's no baseline "thing" whose value would have been constant over such a long period of time to enable us to make a reasonably accurate comparison. Think about how different our society/culture is from the antebellum US and you'll start to get an idea of how difficult/pointless it is to try to compare the "price" of something then to today's currency/pricing. It's basically a totally different culture, which valued people and things in totally different ways. There's no real common ground with which to make a comparison.