[Heathrow express][1] tickets are priced according to peak (7-10, 14-19 M-F) and off peak times. The off peak costs £22 for a single ticket. The tube also has peak/off-peak times/prices (6:30-9:30, 16-19 M-F).

You're cutting it very closely as you'll need time to leave to plane, (if applicable) go through customs, pick up your luggage, and then the ride to Paddington is about 15mins. I always buffer 45-60 mins if customs is included in a large international airport.

Depending on which line (Circle or Hammersmith) you take to Euston, the [underground takes 15-20 mins][2] if you know exactly where you're going. 

Another local may have a more direct route but as a past traveller taking Piccadilly directly to Euston (one tube station change) is about 50-60 mins. It appears that the Piccadilly line is currently affected by delays of [emergency repairs][3].

Note that if you're unfamiliar, to add buffer to allow for turnarounds or asking for help.


  [1]: https://m.heathrowexpress.com/PurchaseTickets/TicketSelection
  [2]: https://www.google.ca/search?q=london%20tube&ie=UTF-8&oe=UTF-8&hl=en-gb&client=safari#fpstate=luta&ltdfid=0x48761ab2b05500a7:0x749d07ad72bbbe13
  [3]: https://www.google.ca/search?q=london%20tube&ie=UTF-8&oe=UTF-8&hl=en-gb&client=safari#fpstate=lut-st-leaf