Origin: Latin suffix -ive
Inexpensive has 2 different meanings across 1 category:
relatively low in price or charging low prices
"it would have been cheap at twice the price"
"inexpensive family restaurants"
low in price
"The second-hand bicycle was inexpensive enough that I could buy it without saving up for months."
The word inexpensive is formed by adding the prefix in- to expensive to mean not costly. It entered English as a straightforward negation of its root adjective without any shift in core meaning.