First of all, I'd like to strongly advise against paying any bribes, to anyone, anywhere in Africa.
I spend a lot of time travelling around in Africa and I've never ever paid a bribe. If you do, you'll make it harder for the next person that comes through, as a bribe will come to be expected. Yoiu might also get yourself arrested, and spending any time in an African prison is not going to be a good experience.
Rant over. Now, what you need to live on. I spent 6 weeks in Ghana in 2010 with my bloke and this is a breakdown of what we spent, which should help to give you a rough idea of costs.
Sleeping - an average of £11 a night for a double room with bathroom.
That's fairly basic, but not uncomfortable, you can go cheaper but we're now in our 50's and need a decent sleep! I don't recall seeing any campsites, but maybe some places would let you put a tent up in their gardens.
Food - our next biggest expense, around £8 a day (3 meals daily, 2 people).
Beers are roughly £1 for a large bottle, and it's good, so we drank quite a lot of it.....
Transport - public transport is cheap, crowded and slow. We used buses, shared taxis and trotros (minibuses) and spend £100 in total, moving every 2 or 3 days and travelling the full length of the country, up the east side of the lake (Ho, Hohoe, Yendi) across to Tamale, up to the Burkina Faso border at Paga, then down again the other side of the lake (via Kumasi) to the coast for a bit - so quite a bit of travelling.
Add on another few quid a day for all the other stuff - entry fees, souvenirs and so on - we spent about £5 a day between us.
So, you can work out how little you can manage on, but unless you are desperate to get away asap, I definitely agree with marc_foo that the more you can save before you go, the better.
You never know what might crop up and it would be a shame to miss out on any opportunities just because you didn't have the cash.
Even more importantly, you don't want to be coming home completely broke and with no job, so some back-up savings would be handy. If you enjoy the job you have now, it's worth asking before you leave if they'd be prepared to employ you again after your trip - that worked for me for about 18 years, taking a few months off each year and working for the same company in between trips.
One last thing, brush up on your French before you go, Ghana is the only English speaking country in West Africa, other than the Gambia, which is not many people's favourite African country anyway.