This is so old school I hesitate to even post this up, but Onyxia is an easy 50-100g for less than 10 minutes of work. She drops around 50g and then her loot can either be vendored or sold. Normally, I would say I make around 50g on her gold looted, then around 10g on vendor trash (the epics), and around 20g or so on the BoE rares she drops. So I enjoy this fight and try to hit her as time she comes off cooldown for me.
I go in with retribution aura, seal of light (I could probably go SoC, but just haven't done it yet), blessing of might, and judge light. I use the normal FCFS rotation, and almost never have to judge wisdom, even with using consecrate. As far as I can remember, I don't think I have dipped below 75% hp on any of my tries, so there is no excuse for dying in here. If you are level 80 paladin and cannot solo this, time to re-roll.
Phase one just run in and tank her with your back to the back wall (this prevents knockback). At 66% she'll launch into the air at phase 2. Chase her down and continue to melee (yes you can 100% melee her for this). Whelps will spawn, but don't even bother targeting them (they peck at you for almost no damage), just stay on Ony. Hit a consecrate and a few DS and the whelps will go down. Phase 3 hits at 33% or so and shes back on the ground, so tank her with your back against the back wall. She'll hit you with the occasional fear, so just run back to your tanking position when it wears off and you'll be fine.
Simple fight, great gold/loot. Happy hunting!
Khor
PS Don't even bother with fire resist here, you won't need it.