Domain Investment is spending your bucks on Domains(Price to acquire, Development cost if applicable, Maintenance cost etc) with the intention of making a profit in future.
Is Developing Minisites kind of Domain Investment?
Yes very much. No matter it is a minisite or a massive site, every buck you spend comes under Domain Investment.
Is PPC the only source of revenue for Minisites?
Not necessary. PPC, Affiliates, Ads etc all can be used for revenue generation.
Can everyone win by developing Minisites?
Not necessary. Be it developing many minisites or developing a few massive sites, it need not work for everyone. You simply have to try to see what works and what doesn’t for you.