To help you plan for transportation during your birding adventure, here is some information about your options:

According to the American Automobile Association (AAA), the average cost in 2018 to operate a motor vehicle was 51 cents per mile assuming a total annual of 20,000 miles driven. This includes cost of fuel, insurance, registration, and maintenance and does not include the cost to purchase a vehicle. Here is a link to the AAA info.
A typical full day of birding in Colorado will average 125 to 250 miles of driving. If you drive your own vehicle, this equals a cost of $63.75 to $127.50 per day.
Renting a car at the Denver Airport picked up late afternoon and dropped off first thing in the morning (with a full day of birding in between) starts around $160 not including fuel. (August 2019 pricing per kayak.com.)
Hiring an SUV & Driver from Execucar (https://www.execucar.com/) with pickup and drop off in downtown Denver is $75 an hour so 10 hour day would be $750!