I think the most important aspect of pricing is that your stated prices are itemized. That way you'll build trust with users.
As for pricing, perhaps a flat rate per request ($1?) that comes with up to 5 minutes of operator time for free. Then any time past 5 minutes could be billed in 5-minute blocks, perhaps $2 each (base price) and tiered up by order price (I dunno, maybe $3 for $500+, $5 for $2500+, $8 for $15000+). Also, you could charge predefined flat rates for any aspects of the order that can be automated.
Again, the important thing is that you're clear about what you're charging and why.
As for pricing, perhaps a flat rate per request ($1?) that comes with up to 5 minutes of operator time for free. Then any time past 5 minutes could be billed in 5-minute blocks, perhaps $2 each (base price) and tiered up by order price (I dunno, maybe $3 for $500+, $5 for $2500+, $8 for $15000+). Also, you could charge predefined flat rates for any aspects of the order that can be automated.
Again, the important thing is that you're clear about what you're charging and why.
Great idea!
EDIT: changed suggested prices (redid math)