airport taxi

from and to WHERE?

"inexpensive taxi"? No. Any taxi that is inexpensive has to be dodgy as hell.

The bus from the STIB 12 or 21 depending on the hours

Why bother asking? Are you trolling us? Train to where and cab from where? You decide yourself based on a route you've not mentioned and with a combination of transport methods no-one has mentioned.
Train and cab combined certainly aren't the cheapest option. Uber, Drive Now, Train, Bus,..

If you do need to take a taxi, taxi2airport are pretty good and always cheaper than getting a taxi from a rank at Zaventem. They wait for you in arrivals and take you to the taxi. We use them to get to Charleroi as well. Much handier than a shuttle bus or rain.