Actually, Miners Haven was open sourced a while back, and a useful module exactly for this was made.
I believe line 113 is the beginning of the code you are looking for.
Code if you don't want to go to github.
local function shorten(Input)
local Negative = Input < 0
Input = math.abs(Input)
local Paired = false
for i,v in pairs(MoneyLib.Suffixes) do
if not (Input >= 10^(3*i)) then
Input = Input / 10^(3*(i-1))
local isComplex = (string.find(tostring(Input),".") and string.sub(tostring(Input),4,4) ~= ".")
Input = string.sub(tostring(Input),1,(isComplex and 4) or 3) .. (MoneyLib.Suffixes[i-1] or "")
Paired = true
break;
end
end
if not Paired then
local Rounded = math.floor(Input)
Input = tostring(Rounded)
end
if Negative then
return "-"..Input
end
return Input
end