So, how much do you think Warren Buffett would be willing to pay for these RBS preferreds?

Let's try to impersonate him and calculate a fair value.
That boils down to the interest rate you'd want to get.
RBS is a deeply troubled institution and these preferreds are deeply subordinated, right behind the government. The government's APS will probably save RBS now, but what is the chance that something will seriously go wrong after all? I don't know and therefore I would want a fairly high interest rate of 15% or higher.

That implies a fair value of $10 for a 6% series.
Some (maybe all) of these preferreds will be suspended for 2 years. To take that into account you need to subtract the present value of the missed interest payments. That's about $3.70 for the 6% issue at 15% discount rate. So, the fair value comes out to $6.30 for a 6% issue.

So, basically, these preferreds are trading far too high right now to be of interest (although you might hold on if you face a large tax when selling).

Actually, I think Warren Buffett wouldn't pick these up even at $6: there is simply too much unknown risk with RBS.