I would add them to a wish list to be notified when they go on sale.

Outside of the random sales that happen, they tend to do a sale run when they bring out a new DLC. So, if we get another one in the future that will likely be a sale window. But, as grampers said, wish listing is the best thing to do. I do that on all the games I have an interest in but wait for sales before getting them.

Instant gaming ?

Check isthereanydeal, you can purchase keys for steam from other sites when they have sales.

cdkeys