Example of creating PayPal button dynamically and getting the amount from user input using their PayPal Button JS.

paypal-js-button.css

body {
		background: #DADADA;
		font-size : 16px;
	}

#price {
 		background: #DADADA;
		padding : .5em;
		border: 2px solid rgba(0,0,0, .35);
		margin-right: 1em;
	}

	.btn {
		text-decoration: none;
		background: #AAAAAA;
		padding : .4em .5em;
		color: rgba(0,0,0, .5);
		border-radius: 3px;
	}

paypal-js-button.html

<label for="price">
	Price
	$ <input type="text" id="price" placeholder="Price" />
</label>

<a href="#" class="btn">Place Order</a>

<script>
	document.querySelector('.btn').onclick = function(e) {
		var price = document.querySelector('#price').value;
  
		var s = document.createElement('script');
		s.type = 'text/javascript';
		s.src = 'http://dl.dropboxusercontent.com/u/19076481/paypal-button.min.js?merchant=C9TYVAV5JMJLU';
		s.setAttribute('data-button','buynow');
		s.setAttribute('data-name', 'Empolyment Express Services');
		s.setAttribute('data-quantity', '1');
		s.setAttribute('data-amount', price);
		s.setAttribute('data-currency', 'USD');
		s.setAttribute('data-shipping', '0');
		s.setAttribute('data-tax', '12.3');
		document.body.appendChild(s);
		e.preventDefault();
	};
</script>