i was accidentally using the previous coordinates to calculate the cost
for each node, which means the A* search was effectively calculating the
cost of the return trip instead of the forward path.
doesn't affect the path at all - the start and end nodes are fixed, so
it doesn't matter whether their costs are included or not - but it does
mean that the cost being returned at the end was off by ((value at end
point) - (value at start point)) compared to the expected value.
magical