I would like to know the cheapest sprocket but good quality to buy to single speed my stock Shimano FC-M300 crank set to get rid of the gears and go "1-by" so its a single speed just on the front.
I think you meant your crank is a FC-MT300. That's a triple with 104/64mm BCD, symmetrical 4 bolt and 44/32/22 rings.
Any chainring that fits the 104mm BCD spider will work.
The cheapest option is to just keep the middle ring (you'll just need shorter chainring bolts if you take the outer ring off.)
Personally, as the 740 is only a 9 speed in the rear I would not ditch the large and small rings to make a 1x9, unless you live in a flat area or don't tackle hills and pretty much always ride on the middle ring anyway.
If you are thinking of swapping the rear derailleur, cassette and shifter as well, you will almost certainly be better off selling the 740 and buying a new bike that is designed as a 1x11 or 1x10.
There is currently FC-MT300 and FC-M3000 cranks in Shimano's Acera range (as afar as I can tell from searching in their line up docs). Either way they both have the same BCD and bolt pattern so the advice above is good.