The first version is quite correct:

Can you afford not to take this approach?

The other one is deliberately contrived and would only be correct if the speaker accentuates the contrivance by inserting a pause between "to" and "not" and laying a stress on "not":

Can you afford to ... not!!! ... take this approach?

The first version is quite correct:

Can you afford not to take this approach?

The other one is deliberately contrived and would only be correct if the speaker accentuates the contrivance by inserting a pause between "to" and "not" and emphasizing "not":

Can you afford to ... not!!! ... take this approach?