I am in the same position - can't afford preschool, but don't qualify for the programs that give it for free.
I love the response below. I organize a SAHM playgroup and there are many of us who have started doing a 'preschool' of our own. We have a theme each month and meet once each week for only 2 hours to learn together. We have a letter, number, color, and shape of the week and each parent can work with their child on their own to reinforce the topics. The themes are fun, we are doing plants this month (trees, food, flowers, ect) and next month we are doing Air (weather, kites, planes, space, etc) We are very structured at school and the kids have really learned a lot since we started in September. We do a "hello" song, a lesson, an activity, snack, craft, story, music, and then a "goodbye" song. We make them line up to go to the next 'station' and they've all learned how to behave in a classroom situation. Each mom takes a turn doing one part of preschool each week, so that helps kids get used to listening to other adults. Anyway, message me if you need more ideas. Really, it is more of a time to let them have a structured playdate and get used to the 'school' type of setting. We (the 12 of us) were all feeling the 'preschool pressure', so we started this up.
Also, kids don't NEED preschool if they are learning from you at home. Seriously, as long as you are working with them to teach the basics and they have friends to play with to learn social skills, there is no need to worry. The sub-titles of a SAHM include friend, nurse, housekeeper, cook, mechanic, handy-woman, and TEACHER.... you are probably a great teacher and you don't even realize it.