I bought my first RTW ticket from www.roundtheworldflights.com, aka Global Village, over a decade ago and paid 699 British Pounds plus 70 Pounds in taxes. The grand total was 769 British Pounds which was approximately $1,290 USD for all the flights! I then bought a one way ticket through a student travel agent for $199 to the UK to still keep my total flight price under $1,500 USD!!! I made sure there was stopover in the US so I could end my RTW flight there.
While that was a while ago, but fortunately prices have stayed around the same as you can seen from Global Village's current deals. One of the largest add-on costs is fuel surcharges, which seem to run about 400 pounds per ticket but have been falling recently.You can still find great deals for 1,299 Pounds (approx. $2,100 USD) or less. There are even cheaper deals for 'Circle Pacific' flights which circles destinations in the Pacific, usually Australia/NZ and back to the UK without actually circling the globe.
Tips: Having a departure date between April 19 to June 30 will get the best prices and the tickets are valid for 365 days after departure.
I wrote a more detailed post on "Going Round-the-World" with lots of tips for planning your trip.