Flight via fly dubai will cost you 1200QR if you book in advance however direct flight will cost you 1800-2000QR.

Hotel within down town or Hamra Street will cost you 200 t0 400 QR ( three to four star)

However for 3 days you must be needing minimum 2500-3000 QR for sight seeing tours,taxi and food as lebanon is not cheap country. Plus all sight seeing is far from Beirut so have to spend lot of money in sight seeing/taxi's.

So total trip will cost you not less than 5000QR per person.