There are much cheaper and better value options if you use the Heathrow Connect (it only takes around 8 to 12 minutes longer but is much cheaper) ...
... This option will hardly take much longer than Heathrow Express services. Heathrow Connect is still a very fast service. ...
Just on the issue of Heathrow Express (HEx) versus Heathrow Connect. Two things worth bearing in mind...
(1) The obvious point that the Connect service only runs every half hour whilst HEx runs every 15 minutes.
(2) Heathrow Connect runs through from the Terminals 2&3 station (what used to be called 'Heathrow Central') on to Terminal 4. Meanwhile HEx runs from T2&3 on to Terminal 5 - which is where the OP is heading.
Therefore the total journey time using Heathrow Connect from Paddington to T5 is 43 minutes, which includes a 7 minute wait on the platform at T2&3 for a connecting HEx train on to T5.
Meanwhile on the direct HEx from Paddington to Heathrow T5 the journey time is 21 minutes - so it is half the journey time when using HEx from Paddington to T5 compared to using the Connect. And yes,
a lot more expensive too!
The money saving option is without doubt the Tube - and coming from King's Cross it shouldn't take that much longer than getting over to Paddington then the Heathrow Express.
On the subject of money saving...
... Cambridge to Hayes [& Harlington] would cost £39.20 which is less than the cost to Paddington. ...
swt class 450 also mentions this... this is compared to
£36.20 for the Cambridge to King's Cross Off-Peak Return. This means that anyone coming from Cambridge who wants to get the Tube across to Paddington and back again another day can do so for an effective cost of £1.50 for each Tube journey, cheaper than the £2.40 cost using Oyster/contactless (and even less with a Railcard).