PTC key is about having refs, so just follow those rules:
Rule #1: Tried to get but no success? That's ok - Keep inviting new refs
Rule #2: Got 1 ref? Nice , now keep inviting new refs
Rule #3: got 5 refs? Very well, just keep inviting new refs
....
10. have 1000 or more refs? Not bad, now keep inviting new refs
There is no guarantee that your refs will be active, so why paying for them? Why buying or paying rents for refs that go sleeping after 1-2 days?
TE are basically free, and there are a lot of TE out there, each of them representing another community where you can find somebody interested. Just avoid those with low number of members.
I know it goes slow, sometime very slow but it's free ! Try keeping your money in your pocket and there are 2 reasons to do so: 1.you're here to make money not to spend them and 2. refs are acting the same even if you pay for them or not