As per the official (?) website of the Ghana immigration service:
FEES AND CHARGES 2015
EMERGENCY ENTRY VISA FEES: $150 & TRANSIT VISA:$20
The requirements for an Emergency Entry Permit are stated as follows:
Emergency Entry Permit valid for 14 days may be obtained where on application has been filed on behalf of the foreign national by other national’s host with the Director of Immigration indicating;
- Personal details including nationality ,place of residence in Ghana;Occupation or business to be followed or undertaken while Ghana;
- Particulars of passport;
- Proposed date of arrival;
- Purpose of visit and duration of stay;
- A statement to the effect that the applicant will not undertake or participate in any activities which might offend the religious beliefs or any section of the community;
- In case of a passenger passing through Ghana ,route to be followed; and
- An undertaken by the applicant to return any pass granted.
- Has had a security furnished by bond, deposit or a letter of guarantee by the person who guarantees the foreign national visit.
And finally, Timatic says:
Passengers with a normal passport can obtain a visa on arrival if they have confirmation that a visa has been approved before departure and if they have:
a copy of a visa-on-arrival approval issued (at least 48 hours prior to traveling to Ghana) by the Ghana Immigration Service. The approval must contain both traveler's passport and visa numbers and a copy of the bio-data and photo page of the applicant's passport.
Visas can be applied for at email@example.com .
Passengers with a confirmation given by the Ghana Immigration Service can obtain a visa on arrival. The passenger's reservation must have an "OK TO BOARD" message. An Emergency Visa on arrival letter issued by Ghana Immigration service will be issued to the passenger or directly to the airline personnel in Ghana when issued at short notice.
So in any case, you will need some sort of confirmation that you will be getting an emergency visa or a visa on arrival, otherwise the airline won't let you board.