it IS expensive to mount tires around here. Im lucky, i have a friend who is a mercedes tech that mounts/balances tires for me for free, while hes on the clock no less
all the LITTLE shops around here 7 bucks per rim balanced! go to the little mexican shops, they have the same equipment as the pro's and in most cases more experience too! my friend and his dad own a little tire shop and have the same equipment as goodyear but charge 7 bucks...they also mount 24" tires all day long!
i got my 255/45/18s put on for $5 a wheel. out back my 285/35/18s were also $5. then a week later when 1 of those went flat, my 295/35/18s were also $5 each. the tire guy didnt even charge me anything any of the 3 total times i was there, i just tipped him that much. didnt even know the man either. and it was on one of those new machines that dont put the claw marks on the inside of your rim.
walmart wont do anything bigger than a 17", they also wont do a lower than a 45 series tire.
it did take me a while to find a place to mount them though.