You have the option of setting the amount people pay when they use your Pool Link.
Set Contribution Amount When Creating a Pool Link
Go to the pool you want to create a Pool Link for.
Tap on Collect $.
Tap on Create Pool Link.
Follow the steps to create your Pool Link, one of which will allow you to set the amount contributors pay.
Set Contribution Amount on an Existing Pool Link
Go to the pool with the Pool Link you want to edit.
Tap on the Pool Settings button in the top right.
Tap on Manage Pool Link.
Look for the Contribution Settings section then tap on Change and follow the prompts.