One JustAsk member asked this question too when his child was just 9 months old. Here's the answer he received: http://www.education.com/question/age-child-typically-preschool/
 
This article references age 3 to 5 as "the preschool years":
http://www.education.com/reference/article/children-three-five-preschool-years/
 
My experience when enrolling our daughter in preschool was that the minimum enrollment age varied with each school we interviewed. Many also had a requirement for potty training first too.
 
I've included a link below to The Parent's Guide to Preschool, which has a variety of resources you may find helpful once you do enroll your child in a preschool.

In terms of cost, that too will vary by school, and some will offer scholarships. Your city may have a low-income program as well.

Good luck in your search for the right preschool for your child.

Pre-school is ages 3 and 4 usually but it is always better to start them early ecspecially in a Montessori program. As for cost it differs from school to daycares that provide that type of teaching. Try montessori.edu for good info. Individual caregivers locally may use this method. They will often advertise if so and are the same cost as any other daycare. Good luck