We didn't know the gender of the baby when we started planning the nursery, so we designed it with a fairly neutral colour scheme of charcoal grey, green and yellow.
I saw a small wall decal of a tree but wanted to make something larger myself. That kicked off the design for the rest of the room.
I made the tree myself using a wallpaper for the trunk and branches, and a variety of wrapping papers that I had in the house for leaves (more info here: http://sara-ferguson.blogspot.com/). I also made all the bedding and the window blind and refinished an old bookshelf which needed a new coat of paint but was a great size for the room. I love the West Elm zebra-print carpet and the crib was a floor model from Babies R' Us that we painted in a mustard yellow. I also made the bird mobile which I make and sell locally.
If you want to design a nursery with budget in mind, pick a wall colour, furniture and any other larger ticket items like rugs, that you really like and think you (and your child) can live with for a long time. You can easily switch up bedding and change the art to better suit a growing child without having to spend much money later.