I think the on site/offsite dilemma depends on deals at the time and whether you would get further use from annual passes.
I have just done a test booking for you based on 2 adults, 1 child and 1 infant for 4 nights from Monday 9th June - Friday 13th June this year in the Cheyenne Hotel and it came in at circa £780 which included free under 7's, a 25% discount and free half board for the adults and child.
I think you would have to go some to beat or match that allowing for approx £80 a night for an offsite hotel, annual passes for 3 and meals.
The ferry companies usually put out special offers in October and February and I have done crossings for as little as £29 return, but we prefer the tunnel. That is usually around £99 return for us. You can use tesco club card points to subsiidise the fare if you shop there. You will also need to allow around £40 for the tolls.