Ultimately the grill is worth whatever you are willing to pay for it. As you eluded to, grills can be found regularly on craigslist from $25 and up. With that being said, certain grills are fetching stupid amounts of money also. The plum is one of the more rare colors (Heck I would like to have one) and definitely harder to come across than green, but what its worth is really up to you. Sounds like you already have a Performer so you are in a good place with not "needing" it. Figure out what its worth, make an offer and if the seller accepts, enjoy your new grill. Sell you green one and that can also help offset the cost.