We've visited McDonalds at DLP a few time over the years. I would say the food prices are slightly more expensive than in the UK (for 2 extra-large meals and a kids meal came to 23 Euros, or about £20.00)
You can pay by credit/debit card and they are pretty fast in serving.
The only issue I have is that if they miss something or get your order wrong, you have to queue up at the far right counter to get your order rectified (could take 20 mins).