Unity pricing question

I am debating buying unity pro, but my question is every time they come out with a new version do i have to re-buy it? (so if i have unity pro 3.3 do i have to pay for pro for 3.4)

You will only need to buy or upgrade to a new version when it is a complete new version of Unity.

For instance, if you bought Unity v2.0, you get v2.1, v2.2 etc for free.

When Unity 3 came out you would need to buy a new version or possibly pay an upgrade price if you had bought version 2 previously.

So in a nutshell if you buy Unity 3, you will get free upgrades until they release Unity 4.