Getting in and camping is cheap. It's the Chunnel that's quite pricey. Getting there can be done in a day if you've something that can really shift. If you landed at breakfast time and gunned it on a big bike for example or a decent car and shared the driving, you could do it. But remember it's supposed to be a holiday not a race. Plan for a day and a half there and same back. We do one day at the show, because it's all that it's worth, giving you an evening before, day at the show, 1 full day mooching, stay the evening, then leave in the morning, cross the day after and home.
If you've nothing but time then take two weeks off and fit the whole thing in as you pass through.