visa required?
 

Is a passport acceptable in Poland, or is a visa required? If so, about how much?

What country are you coming from?

The State Dept website is

travel.state.gov/foreignentryreqs.html

and is worthwhile checking for every trip to be sure you have the proper documents.

We are starting in Paris, going thru Germany to Prague, and then to Krakow.

EU nationals don't need a visa for stays of less than 90 days (or 6 months if you're British). So, assuming you are French, the answer is probably not.

Tourist visitors from the USA need only a valid USA passport to gain entry to Poland for 3 months. Passport should not expire in that period. Do not overstay or leaving gets complicated. US Embassy is often busy replacing stolen passports...be careful. You can apply for an extension at a Polish Passport office. It may be simpler to leave Poland before the three month period is up and then re-enter. Be sure passport is stamped.