Greetings Kathleen:
I would suggest contacting a professional consulting forester to determine whether your yard trees are worth selling. If the trees are not "valuable" as potential sawlogs, then you will have to contact a certified arborist for traditional tree removal costs. Arborists and consulting foresters' rates vary; therefore, you will need to contact them directly and explain the services you are requesting.
Certified Arborists: http://www.illinoisarborist.org/
Illinois Consulting Foresters:
http://dnr.state.il.us/conservation/forestry/CONSULTING_FORESTERS.pdf
Best of luck!