The following links give examples of model kits with my comments. You may be able to find cheaper models if you search the internet. The information provided here should not be construed as endorsement of any company.
Here's my favorite set - not bad at $14.40 (but shipping is steep!) It's just enough to do what you'll need in this class, and fun too. The model will do the cyclohexane ring flip very well, and comes with 6 green atoms so you can build a good model to see axial vs. equatorial hydrogen atoms. The only cheaper option might be to go in with a group of students and buy a larger kit, then split the parts between you. Look at this site even if you're not going to buy a model - there are some pictures that give you an idea of what you can do with models.
Amazon doesn't have as wide a selection of models as Ebay. This link shows used & new Prentice Hall kits, which are OK but don't do the cyclohexane ring flip too well. The "separator" helps take the models apart, is nice but not crucial. I wouldn't pay more than $20 for a used kit.
One student asked about this model kit. This is a fine set. It has enough atoms to do what we need, and with a little practice, I think it can do the ring flip - but not as well as the others. Other minor problems that you should be aware of: the carbon atoms in this kit all look the same, and it takes some practice to distinguish between the sp3 (tetrahedral) carbons, sp2 (triangular planar), and sp (linear). If you start building a kit with the wrong atoms, you'll have to fix it. Also, I've found it is not easy to get all of the pieces back in the box so the lid closes. That's easily solved with a zip-loc bag.