Cost Data
I believe that getting the price the dealers charge is rather difficult. That said, try the St Louis Sailing web site (http://www.stlouissailing.com/). Under their build a boat option you can get pricing for the various options.
The Yanmar saildrive is not among the options listed on the web site. Since it a "special order" installation on new boats you need to work through your dealer. Retrofit of a saildrive on a wing keel is possible, but again you will most likely need to work the purchase and instalation through the maintenance facility doing the job.
I do have the saildrive option on my 250, it was on the order of 12k
If you REALLY don't like smaller boats, or don't plan on keeping your 250 for a long while, I think you will probably be unhappy since you probably are not going to recover the expenditure. On the other hand, if the 250 is going to suit you for any considerable period of time, and you realize you will most likely NOT recover the cost, it is a nice option to have