It really does depend on exactly what type of spindle, but I have a broken one that is about 2 pounds. I really doubt it would be higher than max 5 pounds though.
Is it something like this one you are looking at ? : http://www.ebay.com/itm/DIY-Air-cooled-Engraver-Spindle-Motor-Engraving-Milling-500W-ER11-DC110V-3-175mm-/131874502172
Reply To: Approximate weight of a 500 watt spindle.