Billy Hatcher and the Giant Egg seems to be a hot title this month. Its price is spiking from $22 avg to $30 and rising. I only noticed this spike from a personal interest to grab a copy of this and play it. I've had a lot of fun playing this back in the day and I'd consider this to be a gem for the gamecube. Billy hatcher was considered a commercial failure selling only 250k copies worldwide.
Interesting to see that just in the last week, the average price went up $4.61, and the number of copies sold went up 48%.
Earlier in the year, even earlier this month, you might have been able to find a copy for $20, but looking at the most recent sold Ebay listings, you should expect to pay closer to $30-35 for a copy right now.
I'd estimate this spike at being at its peak right now from the holidays. I'd expect it to hold its value at around $30 or the high 20s from here on out. If you don't have a copy of this game, I'd still suggest you grab yours now, its a very fun and original platformer. I would estimate the game selling for around $50 this time next year, and being one of the more sought after titles for gamecube collectors.