Oh yes, they were that valuable. For a simple wooden church the bell(s) could cost more than the church itself did, and at the very least it was a substantial portion of the cost of erecting a church. The main part of its value/cost was that of the metal. And this extended long after the Middle Ages - e.g. I wrote a paper (in print soon) mentioning a pair of medieval church bells from around the year 1200 here in Sweden that were melted down and recast into a single larger bell in 1829 - even though they were well aware at that time the bells were medieval. They were not the last bells destroyed, it occurred a lot up to the end of the 19th century, although at least by that time they made efforts to document them before destroying them. But even in the first decade of the 20th century there are documented requests of parishes who wanted to recast their bells, but by that time the authorities had started to reject it. By today's standards it seems crazy to knowingly destroy a medieval artwork for 300 pounds of brass, but it says a lot about the metal value. (of course, even today the scrap value of 300 pounds of brass is hardly pocket change either) In the 16th century when king Gustav Vasa forced many parishes to give up bells to (allegedly) pay off debts to Lübeck, it was an infamous act that caused a popular uprising (1531-1533), known as the 'church bell uprising'. As I'll get to, there were more reasons than just the value that made them popular, though.

So church bells were indeed good war booty, if you had the means to transport them. In this county there were not many such opportunities in the Middle Ages, but starting with the Ingrian War 1610-1617 at the latest, medieval Russian church bells start showing up in Swedish churches, and a half dozen or so remain to this day. Restoration of their lost church bells was even a demand made by the Russians in the peace talks. The Swedish commander Jakob de la Gardie denied stealing any bells, although many looted Russian bells turned up in churches near his estates in Sweden. (he did appear to have paid for some of the bells though, although no doubt these deals were made without official sanction from a higher authority than the local parish priest or monastery abbot) Likewise there are also examples of looted Swedish church bells in Russia today, while the continent is even more mixed about due to far more invasions and border changes going on there.

Naturally many bells did not remain bells but indeed were used to make cannons and other objects - this goes both for captured and owned bells. And cannons were sometimes recast into bells in peacetime. So the survival rate of the bells is fairly low, considering that they were otherwise treated with care, did not wear out (the clapper does though) and were rarely vandalized. Had it not been for looting and re-casting most would've survived until today - and many medieval bells are are still in use.

Besides their liturgical role and such, Church bells played other parts as well. It was often a source of pride and bragging rights for parishes to have more bells/bigger bells or more beautiful-sounding bells than others. It not just displayed wealth - but made it audible. Which is another reason for re-castings and such; parishes were eager to upgrade their bells if there was an economic upswing enabling them to do so. In the case I described from 1829, a pair of small medieval bells were clearly valued lower than having a single new one with better sound. (which records state as the cause for re-casting at the time) Their sound became a matter of identity - people knew their bells and the sound of church bells was enough to tell if you were at home or somewhere else.

Last but not least - and especially in the Middle Ages - church bells were magical. Being within earshot of a church bell meant being within a kind of protected zone around a church. Medieval church bell inscriptions are frequently invocations or blessings (not least "Ave Maria, gracia plena..") and occasionally magic words like "AGLA" (e.g. 1, 2) and other names and phrases otherwise seen mainly on protective charms/amulets. There is also lots of folklore where the sound of church bells wards off evil in various ways, like turning trolls to stone. Or just having effects.. In Dalarna where I lived, several glacial erratics - big boulders in the forest, had legends associated with them claiming they flipped over when the church bells rang. (Given rules against personal anecdotes - Here's the National Heritage Board's documentation on one of them; Norrbärke 75:1)

So although it was the metal value that made them worth looting, there were more reasons for people to want to protect their bells than just being valuable church property.

Additional Sources:

Mats Åmark - Sveriges Medeltida Kyrkklockor, 1960

Ture J. Arne - Ryska kyrkklockor, Fornvännen, 1936, p53