Ken's description of Omni's upgrade price policy can be found here:
http://forums.omnigroup.com/showpost...8&postcount=47
Assuming they follow that policy, and the base price of OmniPlan remains the same, you are looking at a ballpark figure of $50-$60 for the upgrade.
If you're using OmniPlan as a professional tool, and if it currently has the ability to create any value for you at all, it seems like buying now and upgrading later is the rational choice. Ken mentioned on the blog recently that OmniPlan 2 is approaching the start of private beta testing; given that it is a major product revision, my guess is that it would be another Christmas miracle if you could actually buy it then :-) Don't you expect to be able to recover $20 a month by virtue of using the tool?