I have read about narrow-wide chainrings being necessary for single drivetrains, because it keeps the chain more secure on the ring.
A narrow/wide chainring is not the only necessary component. You also need a rear derailleur with a clutch to be able to avoid chain dropping from the chainring.
Not only that, but 1x chainrings need a crank with optimal chainline for the only chainring. A 1x setup is intended to be built such that you can use any sprocket in the rear with reasonable chainline. A 2x setup is built such that the large ring is used with N-1 smallest sprockets and never with the largest sprocket, and the small ring is used with N-1 largest sprocket and never with the smallest sprocket. If you install a 1x narrow/wide chainring on a 2x crank, it won't have an optimal chainline.
- Remove front derailleur, cables and shifter. You might recoup some of the costs by selling the front derailleur and shifter as second-hand parts.
- Remove rear derailleur to be replaced with a unit with clutch. You might recoup some of the costs by selling the rear derailleur as second-hand part.
- Remove 2x crankset, put 1x crankset with narrow/wide chainring in. You might recoup some of the costs by selling the old crankset as a second-hand part.
My opinion is that this probably won't make much sense. Selling used parts is tricky as if you won't find anyone nearby to buy these, you have to pay postage, and the price for worn items you get is quite low anyway.
Also, note that if installing a new chainring or crankset, you may need to replace the chain as well because a new chainring might not work with a well-worn chain. If replacing the chain, you may need to replace the rear sprockets too.
Even if starting from the scratch, i.e. you are building a bike and selecting components, a 1x setup won't save any measurable amount of money. You need a cassette with larger range (more expensive), a rear derailleur with a clutch (more expensive) and one narrow/wide chainring (which costs far more than two ordinary chainrings because narrow/wide chainrings are built by CNC machining and machining the narrow/wide features requires more tooling time than simply creating an ordinary chainring). You save front derailleur cost which is a cheap item anyway, and you won't need the front shifter. About the only type of shifter that is expensive is an STI unit, and if using STI lever for the rear shifter, you probably want another identically sized lever, i.e. you will have an STI lever for the front too. In comparison to STI shifters, mountain bike shifters are ridiculously cheap unlike for example narrow/wide chainrings that cost a lot of money.