Use these tips to have a bash that's low in cost, but high in fun:
Set a spending limit before you start. Create your own piata. Make it an activity with the kids. Pick a free location such as your backyard or a local park. Play classic party games. Change them to fit the theme - for instance, "Pin the Badge on the Sheriff" for a cowboy-themed party. Write invitations by hand or send them out by e-mail. Use items you already own for decorations. Use toys that fit the theme. Make your own cake, or make personalized cupcakes for each of the children. Go for two-liter bottles of drinks rather than cans or juice boxes. Use brown lunch sacks for goody bags and let the kids decorate them. Rather than hire an entertainer, have someone you know come to make balloon animals, paint faces or give a puppet show.