Transportation from Charleroi airport to Brussels central

The airport has its own website with a section on transportation options: https://www.brussels-charleroi-airport.com/fr/parking-access

I cannot find on their website how to change the language to English.

There's a box near the top right of the page that says FRENCH. Tap on that to see if you can switch to English
My tablet automatically translates text.

I tried the airport website and found it quite confusing even though I speak French. I never could get it to switch to English even after I found the link. Maybe it's better to go to the source and look at the train and bus site directly. It's very easy to navigate. Here's the website below. And there are options to Brussels directly from the airport. Just enter Charleroi Airport as your origin point and Brussel Central, Nord, etc. for your chosen station.

https://www.belgiantrain.be/en?gclid=CjwKCAjw7_rlBRBaEiwAc23rhj8usmzUFRDB1V5IQ_fuu0vAUQeyWwVXKEpKiZodTT8Cl_WgtL5wLhoClFUQAvD_BwE

For instance what would a taxi cost?

More than it would cost to fly to Brussels main airport instead.

Maybe this link will be helpful to you https://www.brussels-charleroi-airport.com/en/parking-access#shuttle
You should be aware that Brussels Central, despite the name, is not the main train station in Brussels. The largest station is Brussels Midi/Zuid (South). If your final destination is Brussels, then Central is most likely the best. If you travel further by train, then it depends on which train you want to take.

Thanks all. Some good information!