The pen is expensive; still, I _ it.

Explanation

The correct answer is 'bought' because the sentence refers to a completed action in the past. The word 'still' indicates that despite the pen being expensive, the action of buying it has already occurred. 'Will buy' is incorrect as it suggests a future action, which contradicts the implication of 'still.' 'Have bought' is unsuitable because the present perfect tense is not needed here as it implies an ongoing relevance to the present. 'Buyed' is a simple grammatical error, as the correct past tense is 'bought.'