proportional family visit about my income?

Hello dear reader sir or madam I would like to visit my brother in Birmingham Uk My salary is less than £ 500 a month my brother wants to help me buy the plane ticket to go back and he lodges me for a week I am planning to shell out myself £ 300 cash for a week in Birmingham 1- Is it proportional about my income? 2- What are the papers to prove that my brother will help me on the purchase of the flight ticket and why help me?