It all really depends on what you are after. The cheapest option is the "Luggable Loo" listed above, it works fine, but does not fold down. There are a lot of fold-able potties, but they are expensive and don't do anything the bucket can't except fold up. The bonus of the Luggable loo/Homer bucket is that if you use the bag system, you can still use the bucket for other things.
The Port-a-Potty is the closest thing to a "real" toilet and it may be the best thing to bridge the gap and make the significant other willing to go with you. Many can be had for under $100. They require chemicals and you have to find a place to dump them legally.
So:
Bucket style - Cheap, easy and ready to go. Can be used for other tasks
Fold-able - More money, but take up less space. Not ready to go in an emergency, only useful as a toilet
Port-a-Potty - Again, more money, takes up more space, needs chemicals. But, always ready for use, closest to a real toilet.
Shovel in the woods - Super cheap, packs small, not always an option. Most ladies will tell you where to go if told to use it.