It boils down to one thing really.
You can save time OR money, never both. It all depends what value you place on your time.
I frequently do the maths as well. I live in Weymouth, to go via Dover (tunnel or ferry) the return cost is about £175. that's 200 miles each way for me. If I use Tesco vouchers then the crossing is "free" (yes I know I have paid for them really
but I don't actually have to hand over any "money")
It takes a day out my holiday each way as its a 5 hour drive. However if I look to go from Poole or Portsmouth then the ferry cost (even if I do daytime sailings and save the cost of a cabin) the cost is well over £400.
So, what's the best option? 400 mile round trip to [email protected]
(16 gallons of diesel at about £5 a gallon is £80) and a "free" crossing but a day out of holiday each way, OR a closer more expensive ferry at £450 and a saving of 2 days? £365 less cost versus two days "extra" holiday.
If I am going for a month then the travel to and from Dover (2 days) is worth it as the time to get there and back is a small percentage of my holiday, but if I only have 10 days do I sacrifice 20% of that time to save £365 (which buys a lot of meals/wine/campsite nights etc.) difficult eh??
(I have only taken the fuel cost into account, the other costs are very hard to quantify)
So basically there is no definitive/correct answer.