Timmy I routinely stay in 5* hotels, all over Europe (although not the Costas) and usually pay less than 100 pounds per night. For example, my next trip is to Berlin in a couple of weeks time. There I´ll be staying at the Hilton. The rate I have for a double room is 84 euros. This includes all taxes and charges but not breakfast. Cheapest prepaid rate for the same dates, even on the hotel´s ´lowest price guarantee´ website, is 165 euro.
The secret? None really. Just do some research then use http://www.priceline.co.uk and their ´Name your own price´ option. Before doing so you need to research what people are paying. The best site for this is http://www.betterbidding.com.