Their policy was that if you bought a current version of DO ~month before (or after) the beta of the next major version was announced, you'd get the new version upgrade for free (once out of beta).
So if nothing changes regarding that (and this is speculation!), the earliest best time for you to buy DO12 would be as soon as the DO13 is announced. You'd get DO12 for reduced upgrade price and jump to DO13 for free.
Barring that, I don't know for what kind of discount would DO11 -> DO13 jump qualify for (perhaps equal discount, but one can always contact the gpsoft sales via email for individually-tailored solution).