The less expensive filters are as thin as film (<< 1 mm). Normally, you would mount them in cardboard holders. Unlike resin filters, they cannot be used in front of a polarizing filter (i.e., subject, polyester filter, polarizing filter, lens).
I purchase these filters when I'm uncertain about how frequently I'll use a filter or whether I'll even like it. Once I see that I'm a regular user of the filter, I purchase a resin version. My personal experience is that they are more scratch resistant than the resin filters.
If you don't use a polarizing filter much, these would work well for you.