Do you mean cheap to get there or cheap when you are there - eating out, activities etc? Plus what do you mean by cheap? You don't have to spend much time on here to know that one person's 'cheap' is another's 'hugely expensive, couldn't afford in a million years'.
You might have missed the boat with cheap flights, but try the Skyscanner 'everywhere' option or low fare searches on the airline websites.
Then booking.com etc for accommodation.
As for everything else, Nordic countries are generally the most expensive and eastern Europe cheapest, then Spain and Portugal are often cheaper than France, Ireland and Italy in the middle. But what do you want to do while you're there? If it's museums etc, you can often get city passes that allow free entry to multiple museums and free public transport that can work out quite good value.
It also depends on what you want to do for food. You could eat from supermarkets and street food in expensive countries for less than restaurants in cheaper countries for example.
Or you could see if there are last minute packages that are discounted - Jet2 fly from Stansted and Easyjet fly from Luton, there might be a city break or the Canaries or Costa del Sol could be quite warm if you want a beach holiday.