It's all kinda a gray area. Nobody is sure exactly what they're worth, because we don't know if they will be fixed, or if people who lost them (if they aren't fixed) will be compensated.
1.8 as you know turns the texture of source blocks. They become a one use bucket of water (water block), bucket of lava (lava block) and flint and steel (fireblock). In 1.7 you can still see them as source blocks. So people have been frantically selling their source blocks cheap and people have been buying them so they get the chance to wear a source block before they "go". In come cases I think that possibly there will be a trade in. If this is the case im holding on the source blocks. If they become worthless, so be it. There are rumours that the cost of fireblocks is around 5 diamonds now but I wouldn't pay attention to that.
oh and by one use I mean when fire blocks "become" flint and steel they are still technically fireblocks so when you place it, they disappear.
