Hello Linda - may I respectfully disagree with you?
Some cheap filters are indeed similar to the better ones (which tend to cost more) but there _is_ such a thing as a naff ADSL microfilter; and these do tend to be sold more cheaply.
Sometimes they do not work at all, more often they do connect but degrade the signal thus reducing data rate or causing the 'phone connection to be degraded. Some will pick up local radios - we have all heard passing taxis/police/whatever on our analogue radios. Well, this can happen on dodgy filters. I bought one, once! The ADSL line would drop out every time I sent a text from my mobile. It was fun finding that one, but then I'm a telecoms engineer.
The worst ones (in my view) are the ones that work OK, then fail after too much ringing voltage over time, or a kick from some lighting, or old age; you get the idea.
Unless you have a spare filter and think to try it out before calling for help this could cost you a BT (or other telecom supplier) call-out fee. The 'cheap' filter then becomes rather costly!
So by all means go for the cheaper ones, but do get a spare, do check them both out when new, and swap them again if the line goes sour one day. Might fix the fault and could save a big bill from BT for changing a very small item.
Hope your filter continues to work well, and that you have a spare handy, too.