Welcome to Flyertalk. One-way fares are always more expensive than round-trip fares; it's standard practice. Price your trip as a
multi-city trip, not two one-ways, and the cost is far from prohibitive. Plugging your trip into Sidestep (IAD-ATH on 12 September, IST-IAD ten days later), I get a low fare of $754 all in... on a combination of Continental, Turkish and United. On the outbound trip you connect via Newark; on the return, via Geneva. But there are hundreds of combinations available, most under $1,000. Visit
www.sidestep.com. Good luck.