Not to be rude, I'm still kind of a noob and have only heard fables of these coins. Why would I pay 2.5BTC for 1BTC?
Its a collectors thing. Casascius is the first producer of Physical Bitcoins, he is currently not producing more due to legal issues, so he may not be able to produce more, and most of all, Its a collectors thing

There are 1 Oz gold coins valued at about $1250 strictly on metal value, but are worth thousands of times the value of their metal, due to the numismatic side of it. Same applies with these coins, some people think they will be worth tons of money in the future as some of the earliest Bitcoin memorabila, there are some people who think they their premium will continue to scale, there are some people who want to diversify their assets, and there are just some people that think they are pretty.
All that matters is that there is a market for them, and people are willing to pay 2.5-3 BTC for them at this moment. You should see some of the other coins, the 2012 10 BTC Silver Casascius coin is actively selling for >25 BTC
Thanks for the information, I wish I could afford one but I've only mined 1.7BTC thus far.