If the open jaw doesn't price out well, the other option is to book a RT from IDA to either ATH or IST and to then buy a second ticket for ATH-IST or vice-versa. On one end this becomes a connection, and because you'll be on two tickets you want to leave plenty of time for the connection since you won't be protected on a later flight if your first leg is delayed.
This would look like:
IAD-ATH on first ticket
spend time in Athens and then
ATH-IST on second ticket
spend time in Istanbul and then
IST-ATH on second ticket
four hour or so layover
ATH-IAD on first ticket
I'm doing something similar next week:
ORD-FRA on UA
FRA-IST...cruise...ATH-MUC-VIE...time in Vienna...FRA on LH
FRA-ORD on UA