Depends on what years you are talking about. These bikes tend to change a good bit every couple years. All 3 are good bikes but what I would suggest is doing a search on 250F shootout and include the year(s) you are interested in. As far as price that will vary on used bikes so check cycletrader or craigslist for that. I would say a nice used '06 CRF250R would go for around $3000 - $4000, that will depend on the seller and the condition. You can also go to kbb.com and see what the trade and retail value is for any of these bikes. There are some minor differences between these bikes but they are all capable of putting you on the podium. If you are a novice rider you probably wouldn't even be able to notice the subtle differences. Here is the 2006 shootout to get you started. Just understand that when anyone tells you one brand is better or worse they are just speaking from personal preference and most likely have never even ridden all of the other models. There is no clear cut winner in that class, some do one thing better and some do other things better. You have to decide based on your experience and how you plan to use the bike (racing, trail riding, etc.) which one suits you best.

http://www.racerxonline.com/article/racer-x-tested-2006-250f-shootout.aspx