Cheapest single speed conversion ?

You can use the middle or small chainring on the crank you already have. Use the front derailleur as a chain guide. That will save the cost of a narrow wide chainring.

For the rear, you just need a cog with the number of teeth you need for your preferred gear ratio, and spacers for the rear freehub. You can use the original rear derailleur as a chain tensioner. That would be the cheapest way I can think of.

Below video will give you the jist of it, although he made a 2 speed using the barrel adjuster to shift between the two ratios. Good luck.

https://youtu.be/UEc91AHs63k?si=v5lMQq8IzaGYSdgd

You can try 1×7 with a cheap narrow wide chainring from Aliexpress.