<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
<event name="payment_method_is_active">
    <observer name="Stackexchange_Custom_Payment_Methods_is_Active" instance="StackexchangeCustomObserverPaymentMethod" />
    </event>
</config>

<?php
namespace StackexchangeCustomObserver;
use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkEventObserver;
class PaymentMethod implements ObserverInterface
{
    /**
     * payment_method_is_active event handler.
     *
     * @param MagentoFrameworkEventObserver $observer
     */
    public function execute(MagentoFrameworkEventObserver $observer)
    {

    $writer = new ZendLogWriterStream(BP . '/var/log/t.log');
    $logger = new ZendLogLogger();
    $logger->addWriter($writer);
    $logger->info("BindCustomerLoginObserver calling buddy");

    $objectManager =  MagentoFrameworkAppObjectManager::getInstance();
    $cart = $objectManager->get('MagentoCheckoutModelCart');
    $grandTotal = $cart->getQuote()->getGrandTotal();

    //Keep your condition
    //if(Condition){

        if($observer->getEvent()->getMethodInstance()->getCode() == 'checkmo')
        {
            $checkResult = $observer->getEvent()->getResult();
            $checkResult->setData('is_available', true); //true/false
        }

    //}


}
}