There is no minimum budget required for marketing an iOS game , it all depends on what type of audience you are trying to reach. Different game types need different types of marketing, so it all depends on what sector you are trying to reach.
My recommendation to indie. Do not spend money on marketing if you do not have the money for it. Especially if you doing it for a paid app. Spending $100 on a banner ad that almost everyone ignore will not bring you $100 in equivalent of revenue. Focus on free marketing effort.
I believe that most paid marketing (banner ads, video ads, etc... ) are fairly useless for most small developers. There is value in reminding people about an already established and well-known brand, but not much value in trying to build awareness of a new brand. I'd focus on free advertising (leveraging social media, talking to media, etc... )