The farming algorithm isn’t set in stone yet, but it is thought that you should have a vault around the size of the network average vault size. This hopefully maintains a high degree of decentralization in data storage.
If your goal would be to earn back whatever safecoin your spent for storage, then over what timeframe? I don’t think at this time there is a good estimate of how long this would take.
No that is likely to be very incorrect. You will pay safecoin you buy or farm to upload. The farming algorithm will ensure this is the cheapest storage that people (farmers) are willing to allow. The network will “find” the cheapest storage automatically through the farming algorithm.