Although paid directory links might sometimes be expensive, but there are still a few high quality directories online that have been running for years and having links there could be very great.
I think its effective in increasing backlinks of your site. the more you submit your site to directories the higher the chance your site's backlink increase.
For me it is still an effective strategy for link building. Some say it's an ineffective method of gathering links but based on my experience, it still have a powerful effect in boosting search engine rankings. You just need to do it properly!
you can see if it's worthwhile by checking out your competitor's inbound links and which ones of value are coming from, in particular, free and or paid directories. Once you determine their quality (they must be relevant and authoritative) you can try to go after these directory links as well.