The terms of a business combination can provide that former shareholders of the acquired firm may receive additional compensation based on post-combination earnings or post-combination market share price. Would additional compensation based on such earnings or market price be considered an additional cost of the business combination?

No, they wouldn't.

Explanation:

Any extra compensation to former stockholders of an acquired company which is based on post-combination share price or post-combination profits cannot be recognized as adjustments in the price of business combinations.

The reason for this is that changes in the fair value of contingent consideration (in case something happens) after the company has been acquired, e. g. achieving certain profits or stock price, are not considered period adjustments, therefore they cannot be included in the cost of the business combination (acquisition).