Iron Lore's Titan Quest and its Immortal Throne expansion pack have been on sale quite a bit lately, but I don't think I've ever seen the pair go below $9.99. During Steam's latest "Midweek Madness" sale, though, you can score them both for only $4.99.
Definitely worth a purchase at this price, if only to get an idea of what we should expect from Grim Dawn.