You can easily put together a nice-looking Plasma Caster for around $10. Here's how....
Hit a few thrift/second-hand stores like Goodwill or Salvation Army for toys with interesting parts on them. You may even find a toy that could almost be a good shoulder cannon with nothing more than a couple of coats of paint and a good imagination.
You'll likely spend about $5-7 for your parts. Then, with a handy hot glue gun and a couple of cans of spray paint, you can finish off your masterpiece to match your costume style.
A few bucks worth of thrift store items and ended up with the Atlantean Plasma Caster!! Go figure!!!!
Anyway, I hope this helps you out a bit.
I'm sure others will pop in and offer even more advice on how to do some cool Pred props without spending a ton of money!!
Good Luck with your project and keep us updated on your progress!!!