The price is good, but I believe you should only allow a certain amount of users upgrade per month. This would help limit the amount of users abusing reputation and also limit the amount of Premium members on this forum. It would also make being a Premium user more "rare" in a sense and more appreciated.
Then limit it to an amount that would still allow you to pay for server bills & plugins. And then what you have left over could be used towards merges.
I think that limiting investors would be a good idea.
Their accounts should be reviewed before being allowed to upgrade.
They must have approval by you. This will keep the investor group scammer free, hopefully.
I agree with Phreak. Not just anyone should be able to become a Investor. Must have no scam reports ect... Plus, you must upgrade the price to $10 when I get $5.00 to pay for it haha.