Discovery Mission cost cap

Anyone know why the cost cap for the discovery missions have grown so much?
When the program was first announced the cost cap was $250m - the recent
2006 announcement of opportunity has the cap at $425m. Why?
Why can't they keep the cap at $250m? Surely decent missions can still be
done for that value. And infation hasn't double prices for anything else
over that period. Surely we could do 2 missions for $425m.

Nathan