I have not found anything worthwhile in the $400 range. $800+ you can find quite a bit. The livestrong models start at $800 and I quite like them. We have life fitness gym quality ellipticals at work and the livestrong models compare well to them. I don't think anything under $1500 is junk. If you are a bigger guy who plans on using it for an hour a day for years it will break down more quickly than a $1500 one but proper maintenance is more important than spending thousands of dollars.
The cheapest decent elliptical I know if is in the $600 range. The Schwinn 430 gets good reviews in that price range. I have not tried it, though. Nordic Track ellipticals also get decent reviews and I think they are junk. They aren't smooth and feel cheaply made.
At $400 you are really going to have to hit up craigslist and find something used. I have seen several models of ellipticals on there and you should be able to find a decent $1000 elliptical around $4-500. I bought a Sole E25 used on CL for $200 which was a steal. The Sole is comparable to the base model livestrong (I believe Sole makes the livestrong ones).
You'll need to do your research and make sure the elliptical isn't too old and that it was a decent model to begin with. Newer Soles have a double set of rails on the bottom and that makes for a more stable machine. Be sure to get on the machine and try it out. I would go shopping for ellipticals before you buy anything and try out some of the nicer models. It is important to know what a good quality elliptical is like. Look for a good sized fly wheel (anything under 20 pounds is absolutely not worth it and 25+ is better). Look for an 18 or 20" stride length (if your GF is short 18 is probably better). Look for a good original warranty on parts and labor (a good warranty is an indicator of quality). Look for good brands like Sole, Spirit, Octane, etcetera. I'd stay away from Pro Fitness and Nordic Track.
If the elliptical squeaks that is probably not a big issue. It is pretty common for people to neglect maintenance. The rails on the bottom often get dirty and when the wheels roll over them they squeak loudly. It is often loud enough that you think the things falling apart. Cleaning the rails is important. They should be cleaned off and then lubricated with silicone spray (nasty stuff but really works well - don't try to wash a rag you use near silicone spray. It will just make everything you wash smell like silicone).
Anyway, I think your best bet is Craigs List if you can't spend at least $800. Running outside is certainly an option for most people (well, not me, since I just had ACL surgery and can't run yet but most people). Another option is a stationary bike. A solid one will only run about $500 and you can get a pretty good workout by increasing the resistance or speed. The Schwinn Airdynes are quite nice but they are a bit loud when pedaling fast.