I would guess in the $50 range.  I think they are fairly common.  A lot of people have it on there list of the planes you should start with.  But the #5 isn't one of my goto planes, I have mine setup as a scrub plane (i.e. wide mouth and very curved blade) for roughing work.  The condition and age will effect the price.  You could use something like hyper kittens plane dating flowchart (http://hyperkitten.com/tools/stanley_bench_plane/) to date the plane.  I think Patrick Leach's Superior Works page has dating info as well.  I personally prefer the look and feel of the the pre-WW2 planes myself, but the newer ones will cut wood fine.  

If your plane that needs reconditioning isn't a complete pile of rust, the cleanup isn't usually too bad.  There are tons of internet resources on cleaning them up.  I usually go with lapping the sole on sandpaper on my table saw and a WD40 bath for all the metal parts.  If it is super rusty a few hours in Evaporust does an amazing job.  A lot of people use a wire wheel to clean up the metal parts which I haven't tried yet.