Quick service dining plan: £41.14
Disney dining plan: £58.35 Disney dining plan plus: £70.75
Deluxe dining plan: £88.99
Which would mean if the Disney free dining offer goes ahead, that the upgrade cost will be about £31.00 from quick service (moderate hotels) and about £14.00 from Disney dining plan (deluxe hotels). Keep in mind prices through third party travel agents tend to vary (both up and down). It's likely when Disney start making it an official option it will cost £70.99 for adults and £25.99 for children (3-9). We'll update when we have official Disney prices.
An extra £13-£14 to upgrade one of your quick service to table service doesn't seem like a bad deal at all. Keep in mind with an extra table service meal you'll have to remember the cost of extra tipping.
For those who don't know about this new Dining Plan "Plus" option, click here to read all about it. TLDR: it's the Disney dining plan but instead of 1 quick and 1 table service, it's 2 table service.