I got 105,000 miles out of a set of six Michelin X Radial LT235. When replaced they still had 5/32" of tread depth remaining but I knew I would be encountering snow and ice and didn't figure another 10k miles was worth being stuck or sliding off in a ditch. I bought that set and the replacement set from SAM's Club. Approximately $1050 for a set of six or $0. 01 per mile.
That was 99. 9% highway miles, mostly loaded pulling various fifth wheel or conventional travel trailers. Tires were rotated and balanced approximately every 10-12k miles.
Harvey