There are some good fares to the US and Canada starting in Prague that are available across all the three main alliances of Oneworld, Skyteam and Star Alliance.
Fares start at around £1,100 return for travel from Prague to destinations such as Chicago, Atlanta, Washington D.C. and Toronto.
Skyteam
There’s wide availability on the Delta site (for example). The screenshot below gives a number of routing options for travel to Chicago starting at about £1,250 rtn.

The screenshot below shows the final itinerary.

Fares need to be booked and ticketed by the 21st September.
Star Alliance
Using the Lufthansa website, and their low-fare finder, there is wide availability from Prague to Washington D.C. (for example) right through to next year (31,278 CZK is around £1,060 at the time of writing).

Here’s the final price display for a trip to Washington D.C. with Lufthansa, however do note that you may wish to avoid the United Airlines codeshares unless you’re certain to get an aircraft with the new Polaris seating.

Oneworld
Not to be left out, Oneworld also has some great fares from Prague that are valid across British Airways, Iberia, Finnair and American Airlines.
Again, using the Finnair website’s low-fare finder, there’s wide availability.

The Finnair website will offer the best prices with connections via London with a mixture of BA or AA across the pond coming out to around £1,110.
