I remember buying the XBox version of this game on sale near its launch, so I assume I paid around $40 for it. I've never regretted purchasing this game and it has a permanent position in my collection. This is one title I truly feel is worth the $50 retail price. Great story, graphics, music, sound effects and gameplay. Its amazing to see people pass this game up at the bargain price its at now. I've probably bought a dozen copies of this game as gifts since its launch. The only other title that I've purchased more copies of would be Katamari Damashi, also as gifts.
If for some bizarre reason you don't own this game, you really should do yourself a favor and pick it up and give it a whirl. I guarantee you'll be hooked. IMO, the best version is on the XBox, followed by GCN and PS2. All version are excellent, but I listed them in order of graphic and sound quality.