Yes you make a saving when you buy data bundles.
Mobile networks have different rates for their data and "out of bundle" is usually expensive.
Converting to data bundles also helps you in the following ways:
1. It is cheaper than "normal airtime usage"
2. When you buy data bundles, you specifically set aside airtime for internet access
3. You will not use your data bundles when making calls and sending text messages
4. The higher the data bundles you buy the cheaper your data bundle costs.
How this gives you better insight into how data bundles work with Econet, Telecel and NetOne