The cost for this type of construction is somewhat regionally sensitive. Mobilization costs and distance from a qualified contractor will affect the price significantly. Suggest calling deep foundation contractors that are local to your area. I am in Nebraska and have used this type of foundation. The closest deep foundation contractors is in northern Kansas and Kansas City. Contractors from as far away as Minnesota and Illinois have bid on my jobs in Nebraska.
Unfortunatly the answer to your question is it depends. Caisson (drilled shaft) prices are sensitive to several factors. As SperlingPE mentioned mobilization is part of the cost. If you have a lot of feet to spread this over it won't be to bad. If you have a total of 60' on the project the bid prices will be high. Soil conditions also make a big difference. Clays and silts above the watertable probably won't need to have the hole cased. Granular soils will. Casing the hole adds to the cost.
Your local DOT may be able to help find prices. Even if the Bridge design group dosn't commonly use cassions they are used for signs and light poles. Try to find someone in there estimating department.
In Colorado the 2003 average bid price for 24" caissons, including reinforcing, was $41.37 per foot. The range in prices among low bidders on several projects was $32 to $50 per foot.
