You will find prices far below what you will pay in the US. In fact you can eat very well even in Athens and touristy islands like Santorini, if you go to where locals tend to go rather than tourists.
A great place to eat in Athens is in the Psiri District where locals tend to outnumber tourists. Authentic Greek food at great prices.
A classic street food in Athens is a Gyro at 2/euros which is a bargain. I've had them in the US that are over priced and don't come close to the quality you get in Greece.
T-shirts will be sold all over Greece. There will be numerous tourist shops with tacky stuff but every now and then you'll find a good shop with quality products. Prices will vary from shop to shop so don't buy the first one you see or in fact the first of anything you see until you see other prices.
There are quality Greek products including soap, skin lotion, packaged food, etc.
To save on eating out all the time I stop at local markets and buy cheese, olives, baked goods, etc. and have them for lunch. For the most part I only eat out at night, but occasionally will pay for lunch like a Gyro and Beer for around 5 euros.
Here are some links to help with food and souvenirs: