Unity pro price one time only?

If I pay the $1500 to buy Unity pro version (lets say Unity 4 for example), will I have to pay another $1500 for Unity 5 in the future? Or will I be able to upgrade to Unity 5 for maybe a discount or something?

Not really sure how that works. Thanks for any help.

Small updates are free e.g. 3.4 → 3.5. Major updates are discounted [50% off of normal IIRC?]

Ok thanks.