Condition and back stickers would matter, but on average maybe $40-50 for Pokemon Gold and $20 for Mario Tennis if I recall correctly...it's been awhile, don't take my word solely.
edit: I just checked, and there are two Mario Tennis GBC NFR on eBay right now, just fyi.
Sold a Mario Tennis a few months ago for $20 without back sticker to help you out. If you have a back sticker on either of them, then expect much higher (x2 of price at least)
These prices have been steady for a few years. Pokémon Gold is $50 and at most has hit $100. Mario Tennis is $20 not a title that's going to sell quickly. This is going off front labels being clean. If the gold is all scratched up count on -$50.
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edit: I just checked, and there are two Mario Tennis GBC NFR on eBay right now, just fyi.
Sold a Mario Tennis a few months ago for $20 without back sticker to help you out. If you have a back sticker on either of them, then expect much higher (x2 of price at least)
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