I really want to get into learning Capo Ferro rapier fencing, but I can't afford the kind of practice rapier that most people would use. How well would a practice épée work? I found a couple for around 35 dollars, but I don't want to get one unless I know I'll be able to learn at least the basics of Capo Ferro with it. Is there any épée that someone might recommend for this?

The problem with epee would be the same as with any modern sports rapier - they are too short by far, they are designed to handle completely differently from weapons used in the Capoferro's book. When we started doing his style of fencing, we did not have access to any decent training rapiers either, so what we did was to make a handles like the ones on 17th century originals on our own, then mounted the modern fencing rapier blades on them in such a way as to have maximum possible length and they served well enough for something to start with, even though weight and balance is totally off. I would suggest waiting and just gathering money to buy one of training rapiers - there are several makers who sel them for afroadiable prices.

You could learn fundamentals of stance and distance with a hardwood dowel of the proper length. As a step up, Alchem has a simple hilt with variety of blade types (lengths up to 40") for $165. http://alcheminc.com/aguirre.html

Technically, you can learn the basics of fencing with anything (Sean's suggestion of a hardwood dowel being a perfect example of how you can start out cheap). Footwork, lunging and basic positions require no sword at all at first.

You can, in theory, get a rudimentary grasp of the handwork with an epee. The flexible blade means you can lunge at a wall target and get the feedback for your distance based on the flex, and it allows you to train with something where you can visually see the point as you learn the actions. In the end, though, you'll find a modern epee and a properly made practice rapier worlds apart. Even the cheaper practice rapiers (such as the Hanwei "Practical Rapier") will be quite an eye opener in terms of the heft, balance, and even the feeling of the grip.

So if you don't have any choice, an epee is better than nothing. But I highly, highly recommend you go with a tool that was designed for the job, and an epee simply isn't designed with rapier fencing in mind.

An epee is too short and too light compared to a rapier. If you tried using an epee against someone with a rapier, you will be outreached. You will also find that your epee will be easily shoved aside and that you cannot do the same to a rapier.

If you ever get the chance to go to a military antique show and there happens to be a rapier, ask the dealer if you can hold it. It's an eye opening experience. You will notice that antique rapiers have weight behind them, yet are balanced. The rapier will also feel that it has the authority to move aside an opposing blade to enable you to deliver a thrust.

AT best, an epee is a bastardized 18th century small sword or a 19th century dueling sword and is nothing close to a rapier (neither do the two weapons I just mentioned).

based on experience from our own fencing group: using modern sporting epee with you can learn basics about the stance and techniques; you can do a little bit more it you mount a modern sporting blade on something more similar to historical hilt (with cross-guard); and its even possible to do some sparring techniques, provided both sides use the same kind of blade. but to learn the original techniques and learn them right, sooner or later you will have to start practicing with something that has its length, weight, balance... close to the original weapons. end then it will take some time and effort to get rid of habits you got with shorter & lighter weapon.
so, if you are only trying to find-out if "historical fencing" might interest you, going a cheaper way is OK. at least you wont end up with 150 USD in something that you wont use.