I actually did plan it that way! That's a big reason why we're doing PEI after Nova Scotia, instead of before it.
Although the cost difference is only about $30. My understanding is that passengers are included with the vehicle rate, so it's $70 + $5 fuel surcharge for the Ferry vs. $45.50 for the Bridge. Still, $30 is enough to buy a nice lobster supper while on the Island!