Interesting topic. Like TychosElk mentioned, I think their wealth involved land, to which King Philip the (?) was indebted to them and forced their decline through the Pope at that time.
Like all noble causes they grew too big and famous. See the Battle of Montigard (sp). {recently saw a fairly good documentary on this; the Rangers of old - they kicked ass}
And weren't they supposed to have a relic of the Crucifixion? There's your wealth.
Philip IV of France. And yes, their downfall was almost entirely due to them becoming too big, too powerful, and too much of a threat to the status quo. While they served a military purpose they were useful, but with once it became clear that the Holy Land wasn't going to be retaken for Christendom, the established ruling elite were inevitably going to look for a way to get rid of them. And not just because of their wealth and worldly power: they were a threat to the establishment on spiritual terms too, as 'Knights of Christ' who might be seen as having more legitimacy than a royalty which by and large held power through brute force. Hence the charges laid against them: not just homosexual conduct (an easy charge to lay against a secretive monastic organisation, since it was more or less impossible to disprove), but heresy of the most vile kind. They had to be destroyed, and in a manner that gave those responsible for the destruction grounds to claim to be doing 'God's work'.
As for relics of the Crucifixion, the Templars certainly claimed to have a fragment of the True Cross: but there were many such in circulation. I've seen it suggested (not entirely in jest) that there were enough True Fragments of the Cross in circulation at the time to have crucified a 400-foot-tall Messiah. Religious relics of the time were generally only as significant as the people who held them, and as the holders power waxed and waned, so did that of the relic. Once the Templars had fallen, the authenticity of any relic would have fallen into question, particularly given the charges laid against them. Anyone coming into possession of such a relic would be risking their own encounter with the Pope's inquisitors should they make it publicly known,one might think. So again, not really a 'treasure' by any material standards. And as for whether an actual fragment of the cross (in the very unlikely circumstance that they had one) would have any particular spiritual value beyond that of any other lump of ancient wood, I'll leave that question for the believers to argue amongst themselves.
