vBulletin is $160 for a standard user license, with $30 each year for updates and support. A lease is $85 per year. So, after the initial payment, it's not a big drain.
If it's too expensive, I'll tell you that I have tried both phpBB and MyBB (and SMF for that matter) and I find that MyBB is the superior board. But you can try all three and take the one live that you prefer.
Actually, that's what I did for the two sites that have a lower amount of traffic.
I didn't want to pay for separate vBulletin licenses for all of them -- at least not right away.
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