mypmpro-change-price-based-on-gateway-percentage.php

<?php
/**
 * This recipe will add a percentage based fee to the initial and recurring value when using Stripe
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_increase_level_cost_by_percentage_for_gateway( $level ) {
    
    global $gateway;

	// set your percentage here
	$percentage = 4; // e.g. 4 = 4% increase

	// Bail if percentage is not valid.
	if ( ! is_numeric( $percentage ) || $percentage <= 0 ) {
		return $level;
	}

	$percentage = $percentage / 100; // convert to decimal

	$increased_initial_payment   = $level->initial_payment + ( $level->initial_payment * $percentage ); // calculate the increased initial payment
	$increased_recurring_payment = $level->billing_amount + ( $level->billing_amount * $percentage ); // calculate the increased recurring payment

	if ( 
		( ! empty( $_REQUEST['gateway'] ) && $_REQUEST['gateway'] == 'stripe' ) || 
	    	( $gateway == 'stripe' )
	   ) {
		$level->initial_payment = $increased_initial_payment; //Updates initial payment value
		$level->billing_amount  = $increased_recurring_payment; //Updates recurring payment value
	}

	return $level;

}
add_filter( 'pmpro_checkout_level', 'my_pmpro_increase_level_cost_by_percentage_for_gateway' );

This recipe is included in the blog post on "3 Methods to Adjust Membership Pricing Based on Payment Gateway" at Paid Memberships Pro here: https://www.paidmembershipspro.com/adjust-membership-pricing-by-payment-gateway/