Good lesson horses are worth their weight in gold. All horses need constant "reminding" of their job. Good riding instructors keep their lesson horses sharp by riding them every day. Different levels of students need diferent lesson horses. A well tuned, sharp and experienced lesson horse usually doesn't put up with a lower level rider very well. They expect the same sharpness from their rider.
Learning on a horse who has issues doesn't make for a good outcome.
Usually there is a wide spread in the cost for lessons--the good instructors with good lesson horses charge more.
Find a instructor and a horse that gives you the best return for your money. Some times paying more is a much better investment.