When you rent referrals, you do so for a month. But then you have always the option to extend your current refs for 30, 60 or 90 more days. Now usually extending a referral for 30 days costs 28 cents with a Standard account. By extending them for 60 days you receive a 10% discount and by 90 days 20% discount (thats 50 and 67 cents per referral respectively).
Now this may seem like a no-brainer, but take in mind that if you're building your Neobux empire without investing a single cent,
90 days extends may not be your best option.
Look at it this way: by extending for 30 days, you're left with more money on your account and you cant rent more refs faster. In the long shot, those extra refs will produce you more money in those 90 days than the 20% discount without renting more refs would.
If you do invest, then this is no problem for you. I strongly suggest you extend for 90 days and keep renting as more refs as possible.