HOW MUCH IS A CARD WORTH?
PSA is the world's biggest card rating company. We would advise you to use PSA own search engine to see what a card has been sold for: https://www.psacard.com/auctionprices
But we're afraid that the investigation does not stop there. What a card has sold for, does not determine the value at this moment in time. We have bought cards that were sold for €1000 in 2021, for just under €100 in 2023. So it is important that you also check out Ebay, to see what people are willing to sell the card for. It does not matter if a card sold for €100 six months ago, if five people are selling the same card for €50 now.
If multiple people are selling the same card, you may see a significant difference in price. Some people just want the money right now, and are therefore willing to sell the card at a low price, while other people add the future increase in to the price of the card, because they don't need to sell right now. So if it's possible, take a look at the average price that the card is going for, not the lowest or the highest price.
Remember that there is a huge difference in price between cards that are rated PSA 9 and PSA 10, so you can not take the price from a PSA 10 and expect the same price for a PSA 9.
A PSA 10 can go for five times the amount as a PSA 9, as some high end collectors only want PSA 10 in their collection
If you are looking into buying cards from young up and coming players, you will also have to take into consideration that the price that you are paying now, even though it may be higher than the value at the moment, will overtime be pocket change if the player actually becomes a legend.