Prepatch I managed to bribe Wales early on as the English for around 6k which was reasonable. Then on, I couldn't bribe anything ever.

You can cheat your way to a good diplomat by using things like give_trait this SmoothTalker 3 with him selected. Add some other traits and he can bribe most things. It's still hellishly expensive (around 15k for a full stack) but at least doable in that case. I simply don't bother in any case. Just build a stack and kill it. It's cheaper and you get some free experience.