I'd be surprised if any of your above options were much cheaper in the great scheme of things. Google ads in particular seem to be horrifically expensive for certain phrases.
true that,Google ads campaigns could be very expensive and I know for sure fortunejack and a few other bitcoin gambling sites were splashing out over 5 btc a month for banner campaigns at almost every btc ad network,too
it was crazy when you just could not find a site without their ads
think they had almost 70% share of all the ads shown for 3-4 weeks at one particular period of spring
for smaller businesses,0.5 btc a week is way too much,if you have noticed the majority of the sig campaigns are either gambling,some sort of an exchange or mixer service they have instant return from their clients and possible rebills
planning your ad campaign depends on many factors though,large companies know their TA,they have experienced managers who decide how to allocate advertising funds and meet the targets so I don't think they skip bitcointalk forum as their ads platform just because they don't know it exists
could be any reason starting from the ease to start a campaign,when you have to get an account,find a campaign manager,escrow funds etc.
maybe,just maybe bitcointalk forum should run their own ads and position itself as a quality advertisement option as well as a source of info,but of course it is up to the owners to decide