There are a couple books that I found that offer at least a partial explanation but more importantly offer some important caveats.
William Linn Westermann's book
The Slave Systems of Greek and Roman Antiquity gives the following caution:
The prices which were paid for slaves during the first three centuries
varied, as before, according to the age, condition, training ,physical
attractiveness, etc. of each slave. The asking prices differed within
each country according to the conditions prevailing in the various
localities. An attempt to compare the prices prevalent in different
parts of the Empire is therefore precarious, at best, in its results.
He illustrates this point by giving the prices of slaves in different locations in the same time period and prices in subsequent years:
At Rome in the period of Augustus 500 drachmas appears in Horace as
a price applicable to a cheap and worthless slave. A clever home-born
slave, qualified as a reader through knowledge of Greek, might be
obtained for 2000 denarii. In Egypt somewhat later a male slave cost
1000 silver drachmas. Another price paid in 5 B.C. was 1200
drachmas. For the second half of the first century A.D. three moderate prices paid for slaves are available which may be accepted as
approximate indications of the custormary price level prevailing at
Rome: a boy, good at imitation, purchased for 300 denarii, a slave
girl of bad moral repute quoted at 600 denarii as a low price; and a
male bought for 1200 denarii. In comparision with these prices
stand the following from Egypt for the same period: a girl of about 8
years bought for 640 silver drachmas; sale in A.D. 85-86 of an
oikogenes, presumably a very young child, at 10 talents, 3000
drachmas copper (==140 silver drachmas)...
He goes on for another two pages but I think you get the point. This is as callous as it sounds but people viewed slaves as hetereogenous goods and thus they differed in price due geographic location, place of origin, looks, etc.
In order to give some context Frank Richard Cowell's book
Life in Ancient Rome provides the following figures:
...To get some idea of what these figures mean they must be compared
with other costs. Cicero spend 25,000 denarii a year to have his son
educated in good style in Athens. For his fine town house, because it
had been built for the millionaire Crassus, Cicero paid 875,000
danarii. Many poor Romans had less to spend on buying their small house than rich people would be ready to pay for a slave; while some
rich folk spent more on a single large fish for a dinner party than
the price of a poor slave.
Yann Le Bohec gives the following figures that might put things into perspective (it should be noted that figures like this and the ones above are often hotely contested among historians. Often there is only one or two documants that have survived and historians have to make extrapolations from these sources; this is not say that any one source is "wrong" but you might get different answers from different authors):
The garrison at Rome is best documented for the end of Augustus'
reign. A Praetorian's salary may well have been 375 denarii per year
in A.D. 6, rising to 450, then to 750 in 14. When Augustus died the
salary of the urbaniciani was probably 360 or 375 denarii, while
that of the vigiles(firefighters) was only 150. Under Tiberius their
pay caught up with, and then passed, that of the legionaries.
I hope that helps answer your question.