Parts kits for MGs have always run in cycles, especially if they are no longer made.
At first they are still somewhat in use or for some other reason obtaining one is hard or next to impossible. High price tag.
Then some user(s) surplus the guns out and the market is blessed with an abundant supply. Prices go down, sometimes ridiculously so. At one time STEN MKII kits were $15. Suomi kits were a free giveaway with any parts kit purchase from one vendor. Stick mag included, if you wanted a drum included you had to pay $12 extra. Brand new Romanian "G" AKM kits were $99 delivered.
Now prices fluctuate a bit as demand and supply catch up to each other, new shipments arrive, easier solutions for rebuilding become available, etc. Prices go up and down accordingly. Another phenomenon is the arrival of the Internet globally. Some warlord selling guns by the ton to Nick Cage sees his $20 rifles selling as $400 parts kits at Centerfire's web site and now his next asking price ain't $20.
So eventually supply starts dwindling, from finally running out, prices getting too high to make a profit on, countries virtue signalling by destroying billions of dollars worth of surplus rifles, etc. Retail prices of course start to rise. Eventually we're back to stage one where they are rare and expensive. Some more than others.
Moral of the story? As long as you are in the lower price stages, buy all you can afford, especially if they are free gifts or stupidly low. Hang onto them as long as you can, eventually they will get to the final stage where your investment pays off. You may only make $20 off that obscure Elbonian domestic designed subgun kit, but you will always be money ahead over time.
Many vendors do the same, setting back a few pallets of each model when they were cheap. Now everyone is sold out and prices have risen. Suddenly "found in a dusty corner of the warehouse" sale ads appear.
