EverythingMe doesn't use much data if you don't search/view cards- once you install EverythingMe and the Smart Folder data is downloaded to your device, very little data will be consumed. If you clear the app's cache this information will have to be downloaded again, though it's a very small amount of data.
Searching via In-Phone Search doesn't use a lot more data than a regular search on a search engine would. If you use/view mostly textual content, the data usage will be low. If you watch videos or play games it will be higher. Note that search background images are updated with each search, consuming a bit of data every time.
I hope this helps, would be happy to answer more questions regarding EverythingMe here or in
support@everything.me .