yeah you were about to get raped big time I paid 40 dollars for the same tires to get mounted and balanced at a mom and pop shop and they did a great job.
I did the same thing at Sears when I had 17x9s with 285s and 17x10.5 with 315s. Sears near me is not cheap but the guys do a great job. They also did them as soon as I went in there and because I wanted to get them that day, put a guy on it right away. They also did a sweet job mounting the weights on the inside of the tire... it was tough to see them at all!
If I recall, 315s required some non-standard parts to do correctly.
It was free, I did them myself, but I have a tire machine, and a balancer... The weights are cheap too, all the tape weights I purchased cost me $10, but I have enough for another 8 or 10 tire change and balance's.