People need to get away from thinking in terms of "upgrade price" and move toward "membership fee".
You are not paying for upgrades to Sonar, you are paying a monthly/annual Cakewalk membership fee. The benefits of membership include (but are not limited to) bug fixes and new/upgraded features to Sonar on a regular basis.
Once you've been a paying member (or paid in advance) for 12 consecutive months you get to keep any upgrades/bug fixes you've been given as part of your membership.
If you have a break in your membership and wish to re-start it, you should expect to pay more (at least initially) for your renewal to cover the cost of all the updates you will get that were published while you were not a member. Otherwise you'd effectively be getting all that for free and everyone else would simply cancel their membership, wait two years, then re-join and expect two years of free updates as well...
There is no X4, or X5. There is Cake.
If you Currently own X3 Pro there is a short term $50 discount on the annual Platinum membership fee.
By joining now you get the next 12 months of updates and bug fixes as mentioned above, but you
also get all the updates and bug fixes they've been working on since X3e was released**. It is NOT a new version, it is X3e with more stuff added, now re-named to just Sonar. This will always be the case moving forward. Until they change it again.
If you choose the annual payment and keep it up each year the net result is almost the same as you've been used to if you upgraded Sonar 7 to 8 to 8.5 to X1 to X2 to X3. The increase in price is offset by getting more included in membership than just program updates.
**
This is what X4 would have been.