A good player kills 2 Lotus and 3 mex with 2 Scythes or 8 glaives.
In the progress, he loses maybe 2-4 glaives or needs to stand idle to heal the scythes which doesn't matter because they already made cost.
425m worth of metal killed and probably 30 seconds = 60m lost on income (or more if they are overdriven or a constructor is nearby).
A stardust will kill more scythes and glaives, but putting a stardust on every 3 mex sets you either back 30 seconds and you risk losing that stardust during build porgress.
The emeny could choose to expand undefended and make 120m per minute more - a second constructor which follows the first and builds defenses on already claimed mex spots.
If 5 Lotus are needed to defend the whole front, you pay 450m for Lotus.
How much should your 450m Lotus be able to kill?