Your customers won't be very happy if you spin the posts. I said the idea is expensive because niche blogs and a quality niche network means, each blog has a unique post, not spinned. More quality you maintain, better results you get. And overtime, authory status as well.
If you get minimum 10 blogs per niche, be ready to create 10 different posts, the unique ones.
I myself found it expensive when I've a in-houce team of more than 17 writers, and many people who does the directory submission jobs in-house.
I've already dropped the niche blog network idea but keeping only health niche with me. It is in demand and giving me a good value. Though it is expensive no doubt.
I'd advise you to start with any one niche, maintain it, promote it, get the return on investment and then invest on other niche network.
Hope the info helps :)