On my blog
SummerCampProgramDirector.com I have a list of 100 summer camp theme ideas. I just started a series of postings called Theme of the Day. I give five ideas for the theme and then rely on others to leave comments with more ideas. The first day was Wild West. There are some neat ideas on that post. Take a look at the blog each day for new ideas.
As for keeping the costs low, some of my favorite things to do with themes are:
1. Changing names of camp games we already play. For example Capture the Flag becomes Capture the Crystal Skull for a Radiers of the Lost Artifact theme.
2. Having the staff create new skits using the theme.
3. Planning crafts that go with the theme. You're going to be buying craft supplies anyways, right?
4. Look for decorations that can be used every year. Buy them used on Craigslist or Ebay. I found an old steamer trunk for $20 on Craigslist. It makes a great treasure chest for our Pirates week.
When you go to my blog make sure that you leave some ideas or ask questions in the comments section. I always respond to the comments, even though it might take me a day or two.