If you do a little research and have a little spare time every evening to manage it, a jarrarium could be a nice cheap alternative for you :)
$5 for a big jar, $2 for some Anacharis (green brazillian water weed)
Get some gravel from a creekbed/stream, and fill the jar with that water. Set in indirect sunlight and watch it explode with life. you'd be surprised how many little creatures live on the mud and gravel in creek water.
Instead of natural critters you can go to an aquarium and buy a guppy or two and some ghost shrimp for under $5