Offering your disk space is called "hosting" or "farming" (In opposition to "mining"). You set up your own pricing, but you have to be competitive in order to get file contracts. Top hosts target to $1-2/Tb/month.

If have access to cheap HDDs, under certain scales and setups you could make ROI in less than a year. Right now the network is still underused, but the usage is growing fast, so probably in half year or 1 year good hosts will be able to host dozens of Terabytes an be profitable. Of course you'll still need cheap hardware (24/7 server-like infrastructure) and good bandwidth. But the advantages, compared to mining, is that electricity costs are minimal and there is not a increase in difficulty over time: if you remain competitive, you keep getting contracts