None of the stories I've seen put the figure at "dozens of millions of pounds". This article from The Telegraph puts the figure at £11.1 million. The article notes that £6.5m of that figure are "police officer pay costs that would be incurred in normal duties" and mentions overtime costs and "indirect" costs, tallying together to an additional £3.8m. Also, if the picture in the article shows a typical guard detail we see at least 4 uniformed officers, not 2.
We should also take into account those 4 officers are not engaged in regular beat policing but the very specific task of waiting for a very specific person to exit a specific location. There's also the very real possibility this is a politically-motivated policing detail with all the visibility and CYA costs that come with it.
The point may yet be valid but to a lesser degree and perhaps not at all just based on numbers. And with the other factors the Assange case may be so unusual that no meaningful comparison can be made.