In cases where non-Splinterlands assets are used, Steem Monsters Corp will value those tokens at market value at the time of receipt.
In cases where DEC tokens are used for payments, then the price will be determined on a daily basis. For convenience the DAO may prepay a large amount of DEC tokens, however the daily rate the DEC tokens convert to USDs will be based on the daily rate of DEC/USD for each day.

Price would have to be determined as mentioned above for accounting purposes.

As far as 2 year cycles go, that's ultimately up to the DAO. The discussions I've had and comments I've read lead me to believe that the majority of people are not looking for another set of cards any time soon.

The proposal covers a 2 year period of development and includes a core set + miniset + other ongoing and developed revenue streams to return value to the DAO. The DAO would also get co-ownership of the IP.