On the assumption that you want to be holding some cash anyway, as part of your overall asset allocation and as a way to smooth out daily transactions, it seems easy to justify leaving the $500 invested to avoid the fee. If it bothers you, and you have other cash accounts, you could invest an extra $500 from one of those accounts to make up for it.
My family has like six different checking and saving accounts, and I'm always pained when I see how much cash we have collectively sitting idle in all of them. A few thou here and there really adds up.