It all depends on your expected results to what you are advertising for. For myself I like to estimate how many PSA's I hope to sponsor and plan to budget an estimated $1.00 per PSA. For example if I believe an ad campaign can yield me 20 PSA's I would budget $20.00 for that ad. In addition I always look at my SFI earnings for the month and plan to also put 20% of that back into advertising.
