One of the factors that most affects both the capability of a lawn/garden tractor, and the price, is the engine.
 
Small riding mowers typically have single-cylinder, vertical-shaft, air-cooled gasoline engines. Lawn tractors and lawn and garden tractors may have vertical or horizontal shaft single-cylinder, air-cooled gasoline engines. Garden tractors usually have horizontal shaft engines, which may be single- or multi-cylinder, air- or liquid-cooled and gasoline or diesel. Virtually all compact tractors have multi-cylinder (usually 3 or 4) water-cooled diesel engines.

Cylinders
Single-cylinder engines are less expensive and are used when less power is needed. Multi-cylinder engines are not only more powerful, but smoother (less vibration). Three-cylinder engines are especially smooth. Multi-cylinder engines are also more complicated, more expensive and have more parts to maintain.

Vertical or Horizontal
The smaller engines used on riding mowers and smaller tractors are typically vertical-shaft (Figure 1). This means the piston moves back and forth in a horizontal direction, turning a vertical crankshaft. Since the crankshaft is vertical, the mower deck can be directly driven with a belt. Most larger engines have a horizontal crankshaft (Figure 2). This design can make the tractor driveline more direct, but it usually requires more power transmission components for the mower deck.

Air- or Water-Cooled
Small, low-power engines are air-cooled. This means the engine block has cooling fins over which a fan blows air to remove engine heat. This cooling method is inexpensive and effective on small engines, but you must be careful to keep the cooling fins clean to prevent overheating. Liquid-cooled engines (Figure 3) cost more. They use a mixture of water and antifreeze circulating through the engine block and then through a radiator. The liquid absorbs engine heat, then air blown through the radiator removes the heat from the liquid. Liquid-cooled engines maintain more consistent temperatures, thus may have longer service life with less maintenance.

Gasoline or Diesel
Gasoline engines use spark plugs to ignite the fuel:air mixture. Diesel engines do not have spark plugs; they have a much higher compression ratio, and the heat generated by compressing the air in the cylinders ignites the fuel:air mix when the fuel is injected into the cylinders. Diesel engines are more expensive, heavier and more noisy. They also have longer service lives and use diesel fuel, which can be cheaper. In many areas, you can by untaxed diesel fuel for off-road use; it is considerably less expensive than gasoline. Diesel fuel is also much safer to work with, since it is not nearly as flammable under most conditions. Fuel cleanliness is much more important with diesels; don’t buy a diesel unless you are willing to maintain a fuel supply separate from other garden equipment and keep the fuel very clean.