Allow SMS without other data
There is no reason why SMS should be tied to your data package.  Turning data roaming off, indeed just turning data off (if you are not currently roaming and merely want to prove this) should still allow you to send and receive SMS messages as these are sent through the voice connection, not the data one - it's exactly how phones have always been able to send a text before anyone ever had data connections.

So the problem is almost certainly something to do with your config or usage, rather than how the SMS systems works.  I would ask you to check who you are sending to and from (are you using email addresses rather than phone numbers in messages, either as recipients or your sent from address?), and whether you have Settings &gt; Messages &gt; Send as SMS *ON*