Say you want to earn from a niche using adsense (PPC) and affiliate marketing (PPA). For eg. niche is "shoes". Within this niche, you want to have many microniches like "sports shoes" "formal shoes" Is it better to do 1 site (eg. www.shoes.com ) with subdomains like www.sports.shoes.com , www.formal.shoes.com OR do many miconiche sites like www.sportsshoes.com www.formalshoes.com
To be more cost effective, i would create 1 site with subdomains, and promote the main site and subdomain links. It would be expensive to promote a bunch of different microniche sites, unless you have the budget for it!