You can be responsive to not have to pay any money.
However, you must be familiar with creating and html concrete5 theme is for that.
Therefore, the
To be responsive to your site in a short period of time,
You are to ask the person who has the technology, the better.

Thank you for this.

A theme makes your site responsive. This is done by specialized CSS styling and (if applicable) the use of jquery.

There are plenty of themes in the marketplace that are responsive. Simply by installing and activating any of these themes will make your site responsive.

Some responsive themes in the marketplace are FREE!! If you go to my profile, you will see that I have some themes in the marketplace, which happen to be responsive, and a couple of them are free!

Hope this helps.

If I chose one of your themes, free or not free, can the information I've put on my current concrete5 website remain intact or do I have to rebuild again from scratch?

Thank you.

Everything will probably be the same as before, assuming the Area names your blocks resides in are the same. If you have built your site on a default core theme, then it would probably work with any of growthcurves themes.

Thank you. Everything is on a Concrete5 default core theme. Thanks.