this explains the surgery somewhat.
and he is almost a 90 pound dog so yeah he is in the greater than 50 pounds category. ((the other document confirms that he is about 88 pounds))
but yeah the surgery ended up being around $3,000

"Like everything in the world, the cost of TPLO surgery varies depending on region and who is performing the surgery. On average, the cost of the procedure ranges from $2500-$4500."
quote from
http://www.tploguide.com/tplo-surgery.php