I'm a upgrade seller myself, nowadays I pay through SIM-Cards I'm limited to 4 upgrades per SIM-Card per day, and 20 per SIM-Card per month. For every SIM-Card you can also pay through Surf pin, however you can only do this 3 times per SIM-Card, for ever. I've got around 10 SIM-Cards to use currently, expanding this week.
What the person above me said, the demand is greater then the supply now. I think 10M is a fair price, I eventually see them go for 12M-15M. There are still people selling for 5M-7M, however I can guarantee you that your safe buying from me. And thats probably the point that people are willing to pay a little extra for a safe trade, not to mention that those who sell for 5-7m are very limited in pins, in most of the cases.