Road racing is run under the UCI (Union Cycliste Internationale) guidelines and specifications, including the specifications of bikes. Although many Triathlon bikes are also used during time trial stages of road racing tours, they actually are not in essence true triathlon bikes. In other words, the majority of bikes used in triathlon races are simply Time Trial bikes used in road racing.
The first true triathlon bike designed specifically for triathlon was the Specialized Shiv which was introduced back in 2011 at the Ironman World Championship. These bikes are designed in such a way as to maximise power efficiency and reduce drag, however the Shiv does not meet UCI specifications, therefore could not be used in a time trial of a road race tour such as the Tour de France. Triathlons are run under different governing bodies to cycling therefore, have different specifications for what is allowed.
For what it's worth, here are the main characteristics of triathlon bikes and road bikes.
- Aerodynamic frame : Large flat or oval tubes reduce the frontal area.
- Steeper tubes : Set at steeper angles so that the more upright seat tube brings the rider further forward over the bottom bracket.
- Steeper steering column : Gives a stiff feel to the front of the bike, makes it more responsive and creates a shorter wheelbase.
- Fixed aero-bars with bar-end shifters : Allows the rider only one riding position, bar-ends are for aero gear shifting.
- Deep section wheels : Furthers the aerodynamic benefit of riding alone.
- Shallow tube angles : Positions the rider further behind the bottom bracket and softens the feel of the road surface.
- Top tubes : Often constructed with thicker sides to dampen road vibrations.
- Dropped handlebars : Provides numerous hand and arm positions.
- Wheels : Can be exactly the same as on tri bikes but are easier to remove because of the longer wheelbase.
- STI levers : Combines brake and gear shifting into the same component.
Many people starting out in triathlon will use road bikes and continue to do so for many years, sometimes even their entire lifetime competing in triathlons. You can buy clip-in tri bars that allow a rider to sit in the tucked aerodynamic position on a road bike.
Riders will upgrade to a triathlon bike once their skills and endurance strength get to a sufficient point where there are significant gains to be made by switching to a more aerodynamic frame, and seated position.
In regards to cost, they are like motor vehicles. You can get a road bike that ranges in price from $200 up to and above $10,000. Triathlon bikes are the same, entry level triathlon bikes are around the $2000 mark with a few exceptions from some of the cheaper brands. Most triathlon bikes with full componentry and wheels will be worth between $5000-$15,000.