Prices usually go up after the 15th of May and then go down again at the end of September. I just checked prices for a flight to Spain from the US and before May 15th the price was under $500.00 roundtrip. The same flight the middle of September was over $800.00. I am not sure if this applies to local flight in Europe but most of the international flights do this as they think the summer months are busier and they can demand higher prices