[Fizzlemizz]

There's also the problem (Edit: as mentioned by Kakjens) with small remainders giving a result of x.00 Mil. so it could be extended too:

--function for beautiful numbers
local floor = math.floor
local strlen, strsub = string.len, string.sub
local function ReadableNumber(num)
    local ret
    local placeValue = ("%%.%df"):format(2) --probably can be improved into "%.0f"
    local prefix = ""
    if not num then
        return "0 "
    elseif num >= 1000000000000 then
        prefix = " Tril" -- trillion
        if mod(num, 1000000000000) < 10000000000 then
            ret = floor(num / 1000000000000)
        else
            ret = placeValue:format(num / 1000000000000)
        end
    elseif num >= 1000000000 then
        prefix = " Bil" -- billion
        if mod(num, 1000000000) < 10000000 then
            ret = floor(num / 1000000000)
        else
            ret = placeValue:format(num / 1000000000)
        end
    elseif num >= 1000000 then
        prefix = " Mil" -- million
        if mod(num, 1000000) < 10000 then
            ret = floor(num / 1000000)
        else
            ret = placeValue:format(num / 1000000)
        end
    elseif num >= 1000 then
        prefix = "k" -- thousand
        ret = placeValue:format(num / 1000)
    else
        ret = num
    end
    local len = strlen(ret)
    if len > 3 then
        if strsub(ret, len-2, len-2) == "." and strsub(ret, len) == "0" then
            ret = strsub(ret, 1, len-1)
        end
    end
    return ret..prefix
end