the 20GB one, you could probably find around $40 now. either new or used. the 120Gb one between $120-$160(depends on where you buy it, some places have it on sale often) 60 GBs around $100.
it's worth it to get the pro. for $100 more(comparing to the arcade), you'll get a 60 GB hard drive, Xbox Live head set, HD cables included(not HDMI cable, only the Elite comes with that), and ethernet cable(if you need one). the arcade doesn't come with any of that.
however you could get the arcade version, then get this.
http://www.amazon.com/Xbox-360-60GB-...=operasport-20
the only thing you'll miss out on is the HD cables, but you'll get 3 free months of Xbox Live gold.
honestly, the Pro is the better buy if you wanted a hard drive, unless you have the money to spend on the Elite.