If ferry costs are included then it will obviously vary considerably according to what time of year and what time of day you are going. I understand that vans are one area where Brittany Ferries is pretty competitive for fares with the likes of Transmanche and LD Lines. As one-way-hire includes a driver, and isn't self drive, the company will probably pick the crosssing that suits it best.
For that sort of money it is probably worth considering doing what we did - buy a cheap van yourself. Our old Transit (£425 with 1 year MoT, and just under 5m long, 2.1m high) cost very little more than a car on BF (or P&O, which was running to Cherbourg in those days) and carried many, many loads in both directions.
As for normal van hire, it depends where you are. I know a couple of companies in NW Sussex (one low-budget, the other mid-range) that will hire you a van and not object to going overseas.
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