I picked up a couple of the Genesis MP15AC’s a few months ago for $130 a piece with DCC and sound. I thought that was a pretty good price. Both of them had a label on the box listing them as Tsunami Sound equipped. I would think that if it had a Tsunami decoder in it, it would have the same label.
I checked out the link you attached. Based on their prices for the SP units I saved $50 a unit. So I guess I did all right. Not sure why the roadname would affect the price so much. I’m pretty sure Athearn lists the cost as the same for all roadnames of the same model.
I bought a $99 Seaboard MP15AC about 3 weeks or so ago. I checked the Athearn part # at Trainworld against the Athearn website and it showed Tsunami sound and it was. I actually bought this because I wanted only the decoder to put into a non sound UP version I had bought a year or so ago. I figured $99 was pretty cheap for a decoder and speaker and I now have a lot of spare parts, motor, trucks etc. I have bought several locos from this vendor and have been very pleased with their prices and service. (no affiliation). I think Trainworld’s specialty is closeouts so the less popular road names are priced accordingly.