A derailleur can cost $10, but it won’t be nearly as durable. Buying a high quality one is gonna cost in the same neighborhood as $100.
Switching is probably a bit of an unecessary faff. But if you want a drivetrain that can be ignored or neglected and never complain, an internal gear is really great.
For a commuter bike it also allows you to fully cover the chain and chainring if you want.
Where a geared bike with a derailleur will need you to put the bike in the smallest gear, and untangle the chain from the cassette. Replacing the wheel back in requires the chain to be properly laced around the cassette and can require dexterity.
Unfortunately derailleurs are necessary to ensure chain tension when the bicycle frame isn’t designed the allow for manual chain tensioning (vertical dropouts).
Also, run tubeless and flats will feel like a thing of the past.
The geyser of glue makes a hell of a mess too. I went back to inner tubes.
I guess it all depends on what you are used to do. I had soooo many flats last year when the spikes started eating through my one year old schwalbe wintertires. 220 spikes makes 220 holes. Had to give up on fixing the holes and change innertube until I could get new tires. Not schwalbe this time. Don't think tubeless would do any difference here though, see enough people get flats with those too and as someone else stated, the glue can get pretty messy.
Having a properly packed underseat bag is essential to any long ride.
Also I have never run tubeless at 25mm. Most folks I see running tubeless have 28mm at the smallest.
Looking at jensonusa, the cheapest rear derailleur is a Shimano Altus 7/8 speed for $22.