The cost for doing the work depends on:
Level: School, High School, University
Extension: Multiple option,short exercise, medium exercise,long exercise
Dificulty: Easy, Average, Hard
EASY: Short or easy exercises cost from 2 to 5 Usd
MEDIUM: Non trivial exercises cost around 5 to 10 Usd
HARD: These exercises cost more than 10 Usd
Method of payment: Paypal