Why does a CHICKEN SANDWICH cost more than Hamburger at fast food restaurants?

every time i've driven upto a fast food place to order my cheep side has made me select the hamburger over the chicken SANDWICH. but as you all know that chicken is very tasty, especially the grilled chicken. and everyone knows it's better for you. so why does it cost more to purchase a healthier meal. and forget about the fish fillet that is out of the question. there must be something with that ground up special patty that never ever goes bad. it must be made with everything but beef. i imagine ground up rodent, serpent, or lizard tastes good as well. but as you all probably know hamburgers taste nothing like the stake you just bbqd over your grill last night. so what is it? and why is it so cheep?