We spent £400 on a caravan in Norfolk. (A "nice" one, 3 bedrooms, it slept 8, but there were only 4 of us, and my kids liked their own bedrooms)
We had 4 days of quality family time and made great memories.
We visited Sandringham house, an NT place, did a boat trip to see seals, went crabbing, did some nice walks, sat on the beach (with our coats on) for a bit, flew a kite, played 2p machines, had fish n chips on the beach, and donuts... played card games in the evenings.
If we had longer there would have been lots more do, that we couldnt fit into the time we had. We didnt even use the pool, or the other activites that were on the site as we were exploring the area instead.
In the past we have had a nice cheap and cheerful breaks on the isle of wight (tonnes to do), Llandudno and Stratford upon avon.