As per my own experience, business visa, cost around 1,800-2,000QR for a filipina. This is the amount that i paid for my wife's visa on a travel agent.

Affidavit of support/letter is very dangerous to show in phil's immigration officer. They will ask other papers to show that the document you have is genuine or duly authenticated. It is better to say that you will visit your friend here in qatar on an invitation, and if the officer ask for any proof, then, show your friends copy of her passport or QID.

On my wife's case, she showed our marriage certificate.