I doubt there is anything that serves as a guide with current prices.
Some general pricing and other advice:
From my short time looking at recent auctions, I can say that finding anything local will be preferred over online auctions as far as price for the US TG16 games. PCE games seem much less expensive and are a good way to go if you can find a converter card or a modded system.
I also noticed some sellers on ebay are posting joke prices that waste everyone's time. There are a lot of recent posts that are good warnings about specific instances where prices have gotten out of hand.
I say check the completed listings for a specific title you are looking for on the 'bay for some guidance. Set your own pricing rules and go from there.
Buy what you like, and don't get games just to collect them. I say play all of your games and enjoy them individually. For me, getting any new game is cause for a mini celebration. Make a want list and read some reviews. Save up your money and determine if you want to focus on loose cards, games in the jewel case with manual, or the elusive boxed games. Don't be a flipper, those people suck and are parasites on any enjoyable hobby.
A few good games to start with that are worth what you will be paying (good at just about any price): Ninja Spirit, Bonk's Adventure, Bonk's Revenge, Military Madness, SideArms, Alien Crush, and many more.
And in the spirit of the forum, OBEY!