Formal preschool is absolutely NOT necessary, even if there seems to be "peer pressure" to get your kids into it, because it seems like everyone does it. It's nice, and my older son loved it (we took him one year), but I wouldn't spend the money again (I wish we had put it in a college fund instead). We did it to socialize him so he would be ready for kindergarten, as his birthday is in September and we wanted to start him the year he turned 5 - not hold him back a year, but you can do the same thing with play dates and outings at the park and such. You can teach her to use scissors, and to color and paint and the numbers/letters, etc. at home. That's basically preschool.
The best thing in the world you can do for her is be at home with her and be her mother and teach her everything you know, and especially to read to her everyday. The preschool we took my kid to (in another state) cost well over $100 a month for 2 days a week, and was closed for every holiday (but there was certainly no reduction in the tuition), and was only 2.5 hours, which included a snack, and outdoor play. So it was a hassle to drive 10 minutes to take him there, then had to haul in the baby, then before I turned around it was time to run back and pick him up (doing it all over again). Then they were always doing fundraising stuff, which I hate. There wasn't much purpose for the fundraising, just doing it for the sake of it.
Come kindergarten, they start with the shapes, letters, colors and numbers all over again anyway, so that really made preschool seem pointless (not to mention that he knew all that stuff before he started preschool - so I didn't make a point of reinforcing what he was "learning" at preschool, so he knew less by the time he finished the year out than he did before he started preschool).
I've known of women with similar-aged children who formed a preschool co-op where they each took turns providing the lessons to the group of children, and held it once or twice a week. There is little cost in doing that, if you can find a group who wants to do it.
Head start is specifically for low-income people who qualify by their income.
Maybe I just have a bad taste in my mouth from the whole experience, but I would say don't waste your money. I really wish we had put that $1000 in a college fund, and I don't plan to send any more kids to preschool. You are the best teacher she could possibly hope for!