Is it possible to write a future purchase price as "current price + appreciation"?

[Sean Dezoysa]

Hi;

I remember reading somewhere that you could write the purchase price on a "subject to" deal as "principle balance on close date" if you plan on taking over the seller's house and all equity.

I'm not interested in subject to deals (yet) but this got me thinking about how to write a future purchase price for an option. Could I simply write the purchase price as "current + appreciation at the end of the option term" and define the appreciation as the Q1 2014 average appreciation percent for our city from next year's LA times?