Planar Ally spells have more concrete suggestions about agreed payment:
100gp/HD for tasks of up to 1 min per caster level,
500gp/HD for tasks of up to 1 hour per caster level.
1000gp/HD for tasks of up to 1 day per caster level.
Nonhazardous tasks have that amount, hazardous tasks typically increase it. Few creatures will take a task that seems suicidal.
Lesser: up to 6HD, 100xp cost.
standard: 1 or 2, no more than 12HD, 250xp
Greater: 1, 2 or 3, no more than 18HD, 500xp
The xp cost and the reduced HD are *very* welcome IMO, since the old spells were grossly overpowered for clerics compared to the risky versions wizards had to use.
Cheers