The chances of it getting lower are about as much as the $70 tickets selling out (and only leaving the $87 flexible fare tickets left.) If the fares drop, you can always call into Amtrak and see about getting a refund for the difference. They may only be able to issue a 75% refund (depending on how the system calculates the refund) but you'd still get some money back.

If you're wanting to travel tomorrow, I'd book now and call if the price drops. Otherwise I'd check a few different dates if you're okay leaving a few days from now and find whatever date is cheapest.