Looking at one on bahn.com for about 100 euros. Is that a standard price or should I expect to pay more if we purchase it at the train station?>

No the full fare on www.bahn.de is what tickets would cost on a walk-up basis (though you may possible save a few euros by buying online as I think to use a ticket window may incur an extra small fee (to discourage folks from using the ticket window!)

If there are special fares it will say that on your www.bahn.de search - if it only says one fare that is full fare that should be the same as at the ticket window - give or take a few euros.

If using the train on 4 days in a month period for inter-city travel (where Lander Cards are not practical) then look at the German Railpass - a 4 day pass offering four days of your chossing in a one-month period costs $242 on a Twin Pass - p.p. or about 40 euros - many discounted fares are 39 euros - so for about the same as 4 discounted fares you get carte blanche to hop any train anytime and if ending the day in Munich or Berlin, etc you can use the S-Bahn systems there that are so useful the rest of the day with the pass.

extra days above the 4th day are just $18/day more or about 13 euros a day of unlimited travel - incuding many K-D boats on the Rhine and Mosel rivers.

So the more days the more viable a German Railpass can become - at some point cheaper than a series of discounted set in stone discounted tickets which must be booked weeks in advance to guarantee availability.