it should be US $35 plus and extra dollar or two service fee if you arrive outside of normal business hours (evenings, weekends, etc.). i've only ever been asked to provide one photo, but it is smart to have an extra on hand just in case.
i've seen Lao officials accept currencies other than dollars, but it is usually at a very unfavorable exchange rate. nice of those officials to even bother, but you'll be better off if you have exact change in dollars. regards.