Once i get a new computer, do I have to pay for it again?

You can pay me if you like but otherwise no. Welcome to the forum.

EULA say you can run at one computer at a time, but you can have it installed on one additional computer. If you upgrade/change your computer, you can install Sonar on new one even from EULA perspective.
 
As other have mentioned, the reality is more relaxing then EULA. You can activate it on reasonable number of computers you have. There was no reports Sonar refuses activation. The mechanism is there to prevent spreading Sonar illegally, not to limit legal users how and where they use the program.
 
Note that depending on the version, you get third party programs and they have own rules. AD and Melodyne allow up to 2 simultaneous installations, but not "relaxing". With Melodyne you should deactivate old copy first (from the computer where it is activated). But it is allowed to "switch" (deactivate on one, activate on another, deactivate there, activate on the original, etc.). AD can be deactivated online from any computer (it does not "remember" nor block deactivated computer from reactivation, like let say Pianoteq, but in EULA there is no explicit permissions to temporarily move the activation).
 
Note that unlike Windows OEM, it is normally allowed to move music programs from old computer to the new one. In worse case you are forced to contact support, but that is not the case with Sonar nor with bundled products.