Can't you buy plain Doritos where you live? In Australia, you can buy "Original Salted" Doritos, which is just oil, corn and salt: regular old Tortilla chips.
Generally not, at least in the US. When we buy plain tortilla chips here, it's usually Mexican. Given the lack of a Mexico on the border of Australia, I can understand why you might not get the same variety of tortilla chips.
Is there price parity vs flavored Doritos?
Usually, exactly the same price. Though the plain ones are often on special, as I guess they're less popular.