There's no need to splash the cash in search of a good endurance bike. The geometry is probably the most important thing in creating a comfortable endurance bike, and in general bikes at the lower end of the market have a more relaxed geometry. This means that you should be able to pick up a comfortable endurance bike for well under a grand.
However, if you want something made out of carbon fibre and featuring the bells and whistles of some snazzy vibration dampening technology, then you can spend a lot more than that. £2,000 / $2500 and a bit of shopping around should be able to secure you a bike with the same frame design features as used by the pros in the likes of Paris-Roubaix and the Tour of Flanders, albeit made with a cheaper mix of carbon fibre and with a cheaper (although usually perfectly good) groupset and heavier wheels.
Spending more money than this will move you up the groupset ladder but might not give you a significantly better frame, so it's worth considering whether this extra money is better spent on other kit, such as some high quality shorts which will greatly improve your comfort on long days in the saddle, or a bike fit which could relieve any aches and pains that you get from riding.