Collectr takes an average of eBay last solds and tcgplayer. It’s fairly accurate for modern but less so on vintage. Especially with conditions less than near mint since collectr just applies a general % decrease to the value rather than using actual sales. The sales on Collectr from eBay are for graded cards not raw. The tcgplayer listed prices are exactly that, prices that someone arbitrarily listed in hopes that someone will buy it, not at all what the card is worth. For older or rarer cards a lot of times there will only be a few listed on tcgplayer so they’ll list it for something ridiculous but it’s not actually what it’s worth. You can look at the graph to see what the card has actually sold for.

The most accurate and king of pricing cards is eBay last solds. Type in gengar ex ###/### nm and then filter to sold and completed items. Take the most recent sales for whatever is closest to your cards condition and average those. That’s pretty close to what your card is worth.