You can add a prefix or suffix to your filter values in the following location: Filtering → Filter templates → [open a filter] → Advanced options.
These options allow you to add a fixed string before or after a filter value. In the example above, we've added a euro sign as a prefix. As a result, Tweakwise indicates in the XML that a prefix and/or a suffix has been set up.
NOTE: Tweakwise doesn't add these strings to the filter values by itself. This needs to be done in building the front-end. Below you'll find an XML fragment that the Tweakwise app returns.
(This is an XML that is returned from the Navigation endpoint. See Tweakwise → Connectivity → Endpoints)
This XML fragment shows that the setting for the prefix is used separately. Note the yellow marked fragments. Next, the values are used without a prefix or suffix. Note the green marked fragments. These values need to be put in the right place in the front-end.