Well, I don't know your time frame (what time, for how long, etc) but switching Greece for Turkey is certainly do-able.
They are neighboring countries, it is not too much of a problem. Where it poses a bit of a problem is when you try to go Germany-Turkey-Germany in July/August, because this is the time all the flights to Turkey are booked solid and prices for flight tickets go through the roof. Even the long-distance buses were booked out when I looked for transport options leaving Istanbul to Germany in summmer 2006.
You might also want to look at visa restrictions for Turkey. Greece is a Schengen country, Turkey is not. Turkey has a bit of a reputation of being difficult for South Americans wanting entry, look into it. Most nationalities can get a visa at the border though.
Cost of visiting Turkey: Plan on taking approx. 35 EUR per day and person. (Greece is 50 EUR per day and person min.)
A good website for planning your visit to Turkey: http://www.turkeytravelplanner.com
Best times to go to Turkey are September/October and April to June.