While 'astronomical' has retained its original meaning of having to do with space and especially the stars there within, but in common parlance you are more likely to encounter a more modern meaning. Astronomical is a fairly common and emphatic way of referring to something that is both abstract and very big, especially a number, cost, or amount.
The first example of this usage that I have found was from 1959:
At the moment he wondered why anyone would want to visit Khatka, let alone pay some astronomical sum for the privilege.
--Andre Norton, Voodoo Planet
N.b., while you can refer to an "astronomical cost", an "astronomic cost", or "costs increasing astronomically", you cannot say "that cost is an astronomy".