Well....
The Condor Liberation (apparently the only boat/ship running this route?) is propelled by 3 x MTU 8000 M71L engines that use 2072 litres of diesel per hour.
Covering the (roughly) 84 nautical miles from St Peter Port to Poole at the vessel's given speed of 39 knots, each engine will consume 4,465 litres of diesel which, at commercial prices of £1.12 per litre , costs 3 x £5,000.80.
So that's £15,002.40 in fuel alone.
Add on reasonable salaries for all the crew, any docking fees, maintenance costs, insurance (which, given the ship's history and reputation, is likely to be pretty hefty!) and all teh other running expenses, you're looking at quite a lot more...
However...
I just had a look on CondorFerries.co.uk and, inputting the same info there, looks to be offering me a return on the same dates for £181.
So who are you booking passage with and do they include a 'happy ending' or something?