How do I change the Fee Amount display in CiviEvent registration?
I was having a similar problem with php 7.2, Wordpress 5.0.3 and Civicrm 5.9.1.
I ended up adding a javascript function to my website's javascript file. The meat of the function is number.toFixed(2) This forces the number to have only two decimal places.

function formatPrices() {
        const price_spans = document.querySelectorAll('.crm-price-amount-amount');
        if (!price_spans) {
            return;
        }

        price_spans.forEach(function(node) {
            const price = node.innerText;
            const data = price.split(' ');
            const symbol = data[0];
            const num = +data[1];
            const new_price = symbol + num.toFixed(2);
            node.innerText = new_price;
        });
    }