Are you travelling directly from the airport? If so which one?
Have you booked your transport/tickets?

Are you going straight back to airport or elsewhere?

I'm just trying to think of where would be sensible places to stop.

I don't know how badly 'out of budge' the ParkInn was, but the Southend-on-Sea Travelodge (no connection to the USA chain) looks well located, and is available on a flexible rate for £59 on the Weds night, £46 on the Thursday. £105 for both nights.

For ultra cheap hostels, I think you'd need to be looking in London, but it would proably be an early 8am train start.

Depending upon how you are planning to travel, ticket prices for travel at different times might outweigh hotel savings, etc.