France can be quite pricey depending, of course, on your lifestyle. If you steer clear of the clubs and bars, you can make it ok. Unless, of course, you can subsist off of 12€ cocktails and 6€ beers. However, you can sustain for a while there if you go on the steretopyical French diet of bread, cheese, cofee, and beer. Maybe the occasional steak frîtes for lunch. I'd list cigarettes, but while they're a part of the French staples, they're pricey. Naturally, once you get out of Paris, you're likely to see prices fall with the mileage.
Prague is definitely easier on the wallet, though because it's become such a hot destination in the last few years since ascendancy to the EU, it's got its korun-burning pitfalls as well, especially if you stick to English-friendly businesses. If you go to the Czech Republic, make sure you get away from the city a bit and explore the more quintissentially Czech - and cheaper - parts. Also, make sure you pay in crowns and not euro, lest you get screwed on the tourist exchange rate.
If you can make it to Spain, I highly recommend it. 3€ for a litre of beer (providing you order in Spanish), wine almost as cheap as in France, solid meals for around a fiver, groceries for a pittance... Of course, you don't get the beautiful summer weather that time of year, but you don't get the crowds, either.
A month may seem like a long time, but in reality, it'll probably whiz by - so make sure you take a good amount of time in the countries you like, if you can keep from running around Europe like a madman. I've found that the longer you stay put, the more you end up saving.