Formula for choosing a winner (weighted)

Hello - I have tasked with choosing a winner for a contest that was run by my company. A contestant has increased odds of winning based on the employees sales total. ie:

• $1,000 raised equals one entry
• $2,500 raised equals two entries
• for every additional $2,500 raised, 1 additional entry is given

Does anyone have any formula suggestions to serve as a solution to randomly picking a winner based on the above criteria? Thank you in advanced!