The amount you buy can't be the only thing you look at. The ONLY thing you should look at is the profit you're making.
You should find as many networks as possible to sell traffic to (especially ones with fallback URLs). Then find 1-3 networks you can buy the traffic from.
Start off by bidding the lowest possible CPM rate and see if you're earning a profit. If you are, then keep using those networks and gradually increase the CPM rate to gain access to more traffic. If not, you either need new networks to sell to or new networks to buy from. The most difficult thing will be finding networks you can sell the traffic to.
But remember that you can sell traffic to other popunder networks or send the traffic to a blog with banners ads and native ads.
The number one thing to never forget - it doesn't matter how cheap you're buying the traffic for or how high the CPM rates appear to be at the networks you're selling to. At the end of the day, the only thing that matters is your profit.