Indeed registering all your music at copyright.gov can get expensive. It’s much less expensive if you register collections of music vs individual tunes.
There are several things to consider:
1) copyrighting your works just makes it possible to go to court and then the court will decide. You cannot sue for infringement unless you have filed your copyright.
2) suing for copyright infringement is expensive - possibly not worth it unless you would be awarded more than ~$75,000. If under that you might pay all of it to lawyers fees.
3) cue vs song doesn’t really make a difference. A trailer music “cue” would be licensed for 10sof thousands of dollars and an artist song might $0.73 on the iTunes Store or be licensed and bring in 10's of pennies on streaming video. It’s more about the exposure the music has. The more it’s heard, and the more iconic / trendsetting it is (or copying something that is).
In the general library world though - there are a lot publishers who sign music exclusively and in the paperwork for that you will see that you actually do a transfer of copyright to the publisher in return for (usual) 50% of up front fees and retention of 100% of your writers PRO royalties. So they have the option to register the tune and you doing it makes things complicated as you have given away the copyright.
That leaves non-exclusive tracks and self published /pitched works. It’s up to you to decide whether or not the cost, effort, and the substantial time it takes to file for copyright is worth it.
It’s hard to say whether filing makes any sense - there is a lot of music out there and clear cut infringement happens in specific cases -vis a vis -Eminem vs Audi, Taco Bell vs the Black keys, the Gaye estate vs.Pharrell, Robin Thicke and T