If you are looking at used acoustic pianos, you can find them from anywhere from a few hundred to tens of thousands of dollars. For under $1000, the market is very much buyer beware. Unless you have access to an expert, or are buying from someone you trust to know the instrument’s history, and who tells you that the instrument has been well cared for and kept in tune, I’d stay away from discount second-hand instruments. An expert’s take can be found at http://piano.about.com/od/buyinganinstrument/tp/buying_used_pianos.htm.
New starts around $3000, but at that price point, the quality suffers, and the instrument may not be able to hold its tune after just a few years, and may also start having issues like sticky keys, and overall a poorer quality playing experience. Figure a minimum of $5000, and more likely closer to $10,000 if you want to get a new piano of a quality that will last for a few decades. You also need to factor in the cost of tuning and maintenance, which should happen once or twice a year. You can find a list of MSRPs (not actual prices paid) at http://www.bluebookofpianos.com/pin.html.
A good quality, well cared for acoustic piano will last for decades, but over that time, you will spend hundreds (at least) on tuning and maintenance. A poor quality or unmaintained piano will deteriorate within a decade, although it may be reclaimable for a significant chunk of money.
With digital pianos, you can find low end full size weighted keyboards new for around $300. At this price, the instrument will have characteristics that keep you from advancing as easily as you like, usually things that limit you in expressiveness. It would probably take you a few months to a year to start reaching these limitations. But it will be reliably in tune, and will probably last as long as a $3000 upright, maybe longer. The more you pay, the better the quality. For $800-$1200, you can get an instrument that won’t limit most casual players, although the instrument quality keeps on increasing as you pay more. I'd expect any decent digital piano to last a decade at minimum, baring things like water spills and unprotected power surges. Check the manufacturer's warranties for their estimates. Maintenance costs should be near zero on a digital piano.
Some digital pianos are reported to suffer from low quality speakers, so you may want to invest in a sound system as well, which could add $100 or so to your cost. But this is optional, and you can make the purchase at any time in the future.