For some reason, regular clay blocks are worth more than stained clay blocks. Regular clay blocks are worth $1/ea and stained clay blocks are worth $0.25/ea. What?
I'm assuming you are referencing the difference between hard clay and regular clay blocks? If that is the case I postulate this reasoning: Harden clay blocks are only useable for building. Clay blocks on the other hand can be used with a high excavation skill to become something else like slime balls. I would say the non hardened clay blocks simply have more potential uses.
They can also become flowerpots or brick blocks, and be placed as building blocks. To improve our ability to build, andrew has made colored wool and colored clay available cheap. I like the choice.