Firstly, what does "11 silver pieces" translate to? If we assume a Carolingian monetary standard, they had 12 silver pennies to the shilling, and 20 shillings = 1 pound of silver. There was no silver shilling coin, so a silver piece was most likely a penny. 11 sliver pieces was therefore slightly less than a shilling, equivalent to about 0.045 lbs of silver.

Which isn't much, really. That's a pretty cheap sword. Now there were two ways to get a cheap sword:

1. get a mass-produced sword, the sort that were churned out for bulk orders placed by kings and other bigwigs who had to outfit armies of men that didn't bring their own swords with them. But that's unlikely in 878. Steel manufacture was very difficult back then, and was quite artisanal in nature. It would be hundreds of years before steel smelting allowed for the mass production of cheap swords. Mass-produced weapons in 878 were more likely to be spears and axes, and most soldiers were expected to bring their own kit with them.

2. get a big knife and call it a short sword. (In 878 this would be something like a seax, cultellus, or demispatha.)

But if you were expecting to get yourself a nice hefty spatha-type long sword for that price, you might be disappointed. Long swords were expensive luxuries in 878.

I will point out that Samuel Pepys paid 12 shillings for a sword in 1669 (April 30, should you care to look it up in his diary), and since shillings in that period were silver coins, the idea that a sword could cost roughly a dozen silver pieces has enough basis in fact that it could be discovered by a television writer doing a bit of googling. But 1669 is very, very far from 878. Steel smelting and mass-production methods made sword production a lot easier, so swords were a lot cheaper in Pepys' time. (Granted, Pepys was buying a small sword, which is much lighter than a medieval sword, but he was also a man of means, and would probably be springing for a fancy dress sword.) Even so, 12 shillings at that time was about 0.2 lbs of silver, almost 5 times what Utred paid in 878 despite the economies that made swords less expensive. So again, sounds like Utred was getting a killer deal.

I will add that the only arms and armour prices I could find from C. 900 were expressed in terms of sheep. (Phillipe Contamine, War in the Middle Ages) Apparently a coat of mail cost 60 sheep, so there you go. This price list suggests that in the 1300s 1 sheep = 1.5 shillings. If the price of sheep didn't fluctuate too wildly between 900 and 1300, that lets us guess that a coat of mail cost about 90 shillings. Once again, Utred's price of <1 shilling for a sword that was a match for that coat of mail is seeming really, really cheap.

As u/BlueStraggler said, '11 silver' in that time and place would pretty much have to be 11 pennies, and that would be very cheap for a sword. This website collects the price of swords from several different early Medieval sources, and converts them to a single currency so they can be compared. The price of livestock from several of the same manuscripts is also included, so you can have some idea about the worth.

The prices vary pretty wildly, and are likely not representative, but all are well over 11 pence: the 'cheapest' cost 60.

Later in the middle ages there would be swords this cheap though. See for instance this comment by u/isabelladangelo who found a reference to a 14th century sword for 6d (her source is 'Standards of Living in the Later Middle Ages' by Christopher Dyer).