The cost of every VA will vary from the skill set, experience and the amount of load you want him/her to do. Web designers for example, are paid $4 to $5 per hour, data entry VAs on the other hand cost $2-3. Candidates from Manila and Cebu, two of The Philippines biggest cities will usually be of higher quality but cost you more. Their costs also usually vary from their experience and the size of the project. The bigger the project, the more it needs expertise, the higher you will need to pay. But as you can see, it is still a lot cheaper and reasonable to hire a VA, as you need not to worry about tax, benefits, insurance, vacations leaves and sick leave pays, helping you cut your operating cost up to 60%.
This is close to what you should offer them as a monthly salary obviously the more you are willing to pay the higher quality you can get-