One minute code job. The amount from the cache is printed to chat, so you could know it through related events and some pattern matching. If you are curious as to the amount before opening it, I am not sure that is possible to be known.
Lua Code:
for i = 1, GetCurrencyListSize() do
local name, _, _, _, _, count = GetCurrencyListInfo(i)
if (name == "Garrison Resources") then
print(count)
end
end