Bubble Bobble featuring Rainbow Islands, which is the full name of what you saw, usually goes for about $50 complete on Ebay. If it's a good price, I would say buy it, and keep it. I love Bubble Bobble.
Spikeout Battle Street isn't exactly rare, but it's fairly uncommon, for an Xbox game, and had a pretty limited print run. It goes for about $30 sealed, so it's not spectacularly valuable.
i sold my complete copy for $50 shipped on ebay. the digital press guide is reliable for collector-to-collector dealings, but with the ignorant folks on ebay one can usually get more for their games.
http://www.digitpress.com/DP/cmf/search.cmf[/QUOTE]
That's the DigitalPress book value. The problem is, that's what they value it's worth among collectors. It doesn't account for day-to-day game buying and selling among average people.
What's written on DigitalPress can sometimes be good to go by, but keep in mind the most important rule of thumb regarding value of anything: Something is only worth what someone will pay for it.