A basic way to calculate a marketing budget. We'll say that a campaign has a fundraising goal of $30,000 and the average perk of $50. This means that 600 perks need to be sold at $50 to hit the goal.
An average marketing campaign, regardless of whether its social media, PR, email marketing, retargeting, etc., has a typical click through rate to a campaign of 3%.
Of the 3%, 1/2 to 1% actually convert and make a donation. I would share a link with you to a spreadsheet that makes this easy, but the nazi's that run this group don't allow knowledge sharing or URL links to be posted in discussion groups.
Too bad because normally I would share a spreadsheet that makes it very easy to calculate that you need to reach a universe of 2,000,000 target customers in order to drive 3% (60,000 visitors) to a campaign URL and hope for a conversion rate of 1% (600 donors) to purchase a product $50 to hit a $30k goal.
The reason social media and PR is so popular is because you can reach a lot of people with keywords and SEO'ed press releases that can hit these numbers at very cost-effective price.