Least expensive plug-in power conditioner that works

Didn’t realize you didn’t necessarily need surge protection.  These Puron filters have been getting consistently very positive reviews for improving sound quality and definitely worth a try.

https://www.underwoodhifi.com/products/puron

Define "works."

+1 for Puron filter.

I wouldn't waste my time with power conditioners marketed towards audiophiles.  But I'm glad people do so that the ones I want are more readily available.  Search your local craigslist for an Ametek unit, a PowerVar conditioner.  They make them in consumer and medical versions, and focus on utility, the protection and lifespan of your equipment.  Since you want one for your modem, these are what I would buy.
Consequently, I put one behind my PC and headphone listening rig and it made my listening rig sound blacker (silence is silent), but then my entire apartment is on a single 20A circuit in an Edwardian house with no ground in the outlets, so it's a candidate for issues to begin with.

Thanks all again for your comments on the topic.  I am intrigued by the Puron AC Power Conditioner - very good user and reviewer impressions. I may buy one and compare to Shunyata Venom Defender and ifi iPurifier in most critical site to see which works best for me.  Will report back.