expired [PC, Steam] Age of Mythology: Extended Edition $10.73, w/ Tale of the Dragon Expansion $12.48 @ Steam
This has to be one of my all time favourite RTS games that I've put hundreds of hours into. The Extended Edition improves upon the original with enhanced graphics and the introduction of Steam trading cards and workshop.
The Tale of the Dragons expansion (released 2016) is meh in my opinion as the campaign story is rushed and the civ as a whole is unbalanced, but I'd still recommend paying the extra $1.75 for the singeplayer alone.
If you want to make the singleplayer extra hard/better I recommend these mods: