Seeing as no one is using the storage service, the best advice would be a guess. I'd say stick whatever you have in there. Don't spend an extra dime. If you fill it up, add more.
Nobody is using it because I set the default price at some absurd value, and nobody has lowered it. The next release is going to have a lower default, and I'm going to encourage everyone to lower their price until they start getting files.
Nobody wants to dump 60 KS on a 100mb upload.
Sia provides a marketplace/platform for storage. How to build their business in this marketplace will be mostly subjected to the business persons themselves. They have to figure out strategies to build their business. For instance, they can offer free storage in the beginning to promote their business and reputation. As long as this platform provides good business opportunities, the business persons will come just like miners are coming to mine because they see the opportunities to make profits.