0.01 BTC per bump and no free bumps. Aside from buying theymos another yacht this also has the benefit of sheer simplicity. No matter who creates how many threads etc - anyone can bump it for a fee.
I won't do pay-to-bump or similar because it removes any sense of fairness or community participation, and basically just makes a page of most-money-wins paid advertisements. (Paying a fixed fee to create a topic there is
possible, though not high on my list, since that'd at least be fair.)
I'm currently still leaning toward some per-topic metric based on merit, though probably not exactly what I originally stated. Limiting who can bump would be easier to implement, but the roughness/immediacy of it makes it more easy-to-manipulate IMO.