It might be a mistake, but it's not only with monitors. Many of their system upgrades are rip-offs too imo.
And they aren't always the cheapest for their own products. E.g. PCBuyIt have the same monitor for ~£15-20 less click here. Their warranty terms are arguably better than Dell as well.
Also note that Dell do a few good deals from time to time - the same monitor was just under £190 (incl VAT & del) only a few weeks ago click here. G