It's in the title, so we might anticipate that the cross of lead is the most important symbol in the book, and it's symbolic in a number of ways. Let's count them, shall we?
1. It's Crispin's only real connection to his mother.
As the one possession of Asta's that her son inherits, the cross of lead is an important symbol of the bond between mother and child and the initial reason Crispin feels such a strong attachment to it.
Father Quinel gives it to Asta's son:
The priest became very still. Then, from his pocket, he removed my mother's cross of lead, the one with which she so often prayed, which was in her hands when she died. (8.81)
Later, Crispin insists, "It's precious to me" (28.31). Of course it is. Not only is Asta dead, leaving Crispin an orphan in the world, but he's never known his father, so Asta was his lone parent, which can only make her that much more meaningful to her son.
2. It's Crispin's only real connection to his father.
Though Asta told her son his father died in the Great Death, it's clear there were many things she wasn't honest with him about, including his own name (you know—since she literally never used it, thereby keeping it a total secret from him). The identity of his father is another one of Asta's secrets. Instead of telling her son the truth about who his father was, Asta wrote it on the cross of lead, knowing her son can't read:
"It says, 'Crispin—son of Furnival.'"
I stared at her.
"You're Lord Furnival's son." (49.19-21)
Say what? That's a pretty big deal, and when Crispin finally learns this truth, everything he's been going through finally starts to make some sense. When he confronts John Aycliffe, Crispin uses the cross to prove his relationship to Lord Furnival:
In haste, I took out my cross of lead from my leather pouch. "It's written here," I said, holding it up. "It was my mother's. Given to me when she died. She wrote the words on it." (56.14)
As Aycliffe approaches, Crispin uses the cross as protection:
In response, I held up my hand, using the cross that rested in my palm as a shield. (56.22)
Finally Crispin agrees to give up the cross, and with it all claim to being Lord Furnival's son, so long as Aycliffe lets him and Bear leave Great Wexly unharmed. Giving up the cross is a symbolic act of giving up any ties to Lord Furnival, who is not in the running for Father of the Year anyway.
3. It's a cross, so we know it's going to be important.
Literature is full of religious imagery, so anytime a cross shows up in a book, take note. The author is trying to tell you something.
In 1377, the Church controls everything, perhaps most especially the thoughts and minds of individuals. It is so infused in everyday life that the very times of day and days of the year are run by the church calendar. Symbols like the cross of lead are very important to the faithful.
However, in the late 14th century, new religious ideas are stirring that will culminate in the Protestant Reformation in the 16th century. Some people are beginning to think that symbols, physical objects, and even formal worship aren't necessary to connect with God. Bear is the voice of these individuals:
"All these things… your cross, your prayers. As God is near—and surely He always is—you need no special words or objects to approach Him." (24.44)
"Crispin, as Jesus is my witness, churches, priests—they're all unneeded. The only cross you need is the one in your heart." (24.46)
The cross, then, stands at a crossroads of sorts between the current dominant religious practices and the burgeoning resistance to the Church's primacy. If we step back and consider this within the broader scope of the book, we can see that this is also sort of true about Crispin: He stands at a crossroads of his own, struggling to make peace with his past in order to go forward anew.
4. What's it worth?
That depends on whom you ask. To Crispin, the cross is something to remember his mother by, but it's also the object that confirms his relationship to a cruel and unjust father in Lord Furnival. To Lady Furnival and John Aycliffe, it's a symbol of the danger Crispin's existence poses. Bear, on the other hand, speaks of the cross in terms of its religious and monetary value.
"I know what it is. It's made of lead. Made in countless numbers during the Great Death. Never blessed, they were given to the dying as false comfort. They're as common as the leaves and just as sacred." (24.45)
Lead is not an expensive metal, like gold or silver, so the cross is made of common material, and it's made only more common by the fact that it's never been blessed, which would give it some value as a religious artifact. However, crosses like these brought comfort to the dying (whether false or not), and Crispin's cross brings him comfort, so it's not entirely without value. And of course, insofar as it proves Crispin's connection to Lord Furnival, the cross is priceless in the eyes of John Aycliffe and Lady Furnival.