The fact is, you really don't need much in the way of sleeping gear for that part of the world in August. You'd probably be just fine with a Cocoon Silk Mummy Liner from Design Salt (US$60; www.designsalt.com), which if you're a warm sleeper ought to keep you comfortable to about 50 degrees Fahrenheit (ten degrees Celsius). You could increase that rating just by taking along a light blanket made with Polartec or similar material for an inexpensive but pretty warm setup.
Of course, taking a light sleeping bag provides that extra insurance just in case it does get a little cool at night. What you need is a synthetic-fill bag rated to around eight degrees Celsius. In the U.S., that list would include the Sierra Designs Delta Fastbag ($99; www.sierradesigns.com), which has an uninsulated nylon "floor." Atop a mattress or pad, it'll keep you warm to eight to ten degrees Celsius. I'm not sure if you can find one of those in Europe, but I know you can find a Lafuma Extreme 800, which should run about 60 euros (www.lafuma.fr). It's an eight-degrees-rated, synthetic-fill bag that's very light (one pound, 11 ounces, or 756 grams) and compressible. Mammut's Kompakt Traveller (US$140119 euros; www.mammut.ch) is a similar bag with a synthetic fill and a temp rating of about eight degrees also. Stuffs down to nothing. You can always unzip and use it as a blanket, too.
Have a good trip!