Personally I think it would be better to have the spray foam on the outside of the batt insulation so you don't have cold air trying to blow through the batt insulation.  You would still need a vapor barrier on the inside but 6 mil plastic isn't that expensive and with the outside spray foamed you wouldn't have nearly as much air movement in your walls.  I saw one building locally where they insulated an existing steel building by spraying 1 1/2" of spray foam between 2x4s the outside steel attached too then they ran horizontal 2x4s between the posts on 24" centers to attach the inside liner too and stuck 5 1/2" batt insulation between them topped with a vapor barrier and it created an extremely tight building at a fairly reasonable cost.