We added white polka dots to a wall in our son's room for under $20. Here are the step by step instructions, good luck & Happy DIY!
Use a compass to draw 6 inch circles on peel and stick vinyl wall stickers - I purchased these at Diamond Vogel
Cut out the circles - our project required about 40 circles
Make a grid on the wall - Jesse placed tiny nails at measured intervals in the corners of the walls and then ran string between them. I wasn't a fan of adding holes to freshly painted walls, but they weren't even noticeable when we were finished.
First, Peel and stick vinyl polka dots at each string intersection
Second, add vinyl dots in the center of each square to make off-set rows of dots.
Remove string and nails, touch up holes if needed. - Voila! Adorable DIY polka dot wall!