Probably a placeholder price in the high end. I don't know about British rules but in Denmark, if you order something for a certain price, even if the price goes up before the product is available, you get it at the price you ordered it for - but if it goes down, you get it for the cheaper price. Thus, shops set the price tag high enough to guarantee that they do not lose money on the product.
PayPals pay in 3 is between PayPal and the buyer, it's transparent to the retailer so they don't know you paid in 3. They don't get charged any fee or anything.The retailer gets the full amount immediately. PayPal will get any interest fees, there's no reason for Simply Games to charge more for pay in 3.
On their site they say up to £2000 can be on interest free credit so presumably no premium under that price.
They may have put £500 simply because if they have to go lower customers will be happy, but if the price was higher they'd lose out because customers would still be entitled to that price so better to guess high.