To be clear, define buying power as the most you would be able to spend on a single card during the buy phase of Turn 4.
(As an example, playing Lighthouse, Bridge, 3 Coppers would give you 6 buying power - $1 from Lighthouse, $2 from Bridge, and $1 from each copper.)
Strictly speaking, in that example you're only able to spend $5. How about defining it as "the listed cost of the most expensive single card you're able to buy"?
Thanks for the suggestion, I like this better so I edited it.
I'm finding there are also a lot of solutions at $9, so let's go for $10+. I didn't see the Envoy + Bank solution coming, that's likely the max we'll see, but other $10+ solutions could also be interesting.