Under 10$? Let's see...
Broken Sword: Twin Pack US: $7.49
Ceville US: $9.99
Commandos 2: Men of Courage US: $5.24
Commandos 3: Destination Berlin US: $5.24
Imperium Romanum Gold Edition US: $9.99
Tropico Reloaded US: $4.99
Commandos 2 (tactical game by Pyro Studios) is famous for its difficulty. Don't know if Commandos 3 is hard, as well.
If you don't like point & click adventures, I'd say out of those get Tropico Reloaded.
It contains
Tropico http://www.metacritic.com/games/platforms/pc/tropico
Tropico Paradise Island http://www.metacritic.com/games/platforms/pc/tropicoparadiseisland?q=tropico%202
Tropico 2 http://www.metacritic.com/games/platforms/pc/tropico2piratecove?q=tropico%202
I never played them, but they seem to be very good strategygames/building sims.
