It's $0, but it is generating an invoice. If I later wait for InvoicePaid, it returns the invoiceid no problem. Of course if I look into the DB, I see the entries for the invoice no problem.
I tried storing the OrderID, then, on InvoicePaid taking that invoice ID and looking up the corresponding OrderID in tblorders but WHMCS doesn't add the entry into tblorders until well after the InvoicePaid hook happens... so there's no way for me to look it up that way.