In reality, slots don’t cost money at all, for 99% of the airports in the primary market (that is, between the airport and the airline). Few airports have tried auctioning its slots (one example was CAN back in 2015, but failed) but the vast majority follows either the first-come-first-served rule, or a pre-defined administrative allocation rule. LHR falls into the latter category.
The price you see on the news are from secondary market (that is, between airlines). This will only happen in airports where slots are extremely scarce. Some regulators specifically prohibit selling/buying slots, but only allows trading slots with slots; in other words, cash cannot be involved in the transaction.
In the notable case of LHR, you can very well tell that airlines selling the slots are those with long history of presence in LHR who easily got the slots decades ago. They are basically making money out of nowhere. If you put such feature in a game, it would be death sentence to new players.