While selecting an instrument to practice on for piano lessons, you have at least three choices: get a keyboard, get a digital piano, or get an acoustic piano, the so-called “real” piano.
Keyboards are the cheapest. If you are going this route, I suggest you get a touch sensitive one with at least 61 or 76 keys (the "real" piano has 88 keys). You could probably get a used keyboard on craigslist for under $100, or a brand-new, touch-sensitive, 76-key keyboards at Target or guitar center for around $200. One advantage of getting a keyboard is that it does not take up a lot of space.
You could consider getting a digital piano. The price will be higher than the keyboard (usually ranging from $600 upward), but the keys will be much more similar to the real piano. Go to guitar center or go online at such website as www.kraftmusic.com and look for a good deal. Sometimes you could get a nice digital piano that is worth $1,200 for $400 when it is on sale (keep an eye for the Labor Day sale or the Christmas sale). Also the digital piano comes with the pedals, which will be needed with more advanced music. One advantage digital piano has over real piano is that it never has to be tuned, and you could hook it up with a headphone and practice with it at midnight, and nobody will hear the music!
You could consider getting a “real” piano, which also comes in many sizes (console, upright, and grand). The bigger the piano, the more power it has, and the more expensive it gets. But, it is possible to get a used piano in playing condition for $500. If you live in the Boston area, you could go to East Cambridge Piano at Somerville, MA and try your hands on one of the pianos.