Pulling weeds is hard work and doing it well and quickly takes skills. I charge my regular hourly rate for weed removal.
If the OP is asking how much to charge, then he does not have the experience to charge the hourly rate that a skilled and experienced crew would charge.
So, there is no telling what a fair price would be or how long it would have taken an experienced crew to do what took two guys 8 hours.
Maybe you should ask your customer how much the job done is worth to them. At least that way you will be able to work for them again.