I don't think there will be any surprise as to how the Apple Watch is sold. It will be just like their computers. The website will say "Sport Edition from $350" "Steel Edition from $xxx(x?)", and "Gold Edition from $xxx(xx?)".
When you click to the page for each edition, just like in the Macs, it will have a base configuration. This base configuration will match the "from" price listed on the front page of the website. It will include whatever is the cheapest strap for that edition. That is to say, just as some Macs start with more powerful CPU and can't be downgraded, the Steel and Gold editions will be made available with a band that is more expensive than the Sport band, and even though you can buy an additional sport band if you like, you can't go below the base price by trying to get ONLY a Sport band. The Sport Edition will obviously start at $350 with a Sport band included. They may charge more for some colors, or they may not.
From there, you'll be able to choose more expensive band options. Presumably you'd be able to buy any band more expensive than the base band, and this will mean your watch will come with ONLY the band you've chosen, just as Macs don't come with the base RAM and the RAM upgrade you paid for, it just comes with the total of the upgraded RAM you ordered (e.g. you order 16 GB of RAM on a computer whose base is 4 GB, you only get 16 GB RAM when the computer arrives, not 16 GB + 4 GB).
Then you'll have the option to buy additional watch bands. This will include bands at all price points, but they will be packaged separately, not in the watch box, the same way a printer you order with a Mac won't come packaged into the Mac box.
There is no way that Apple is going to sell the Steel or Gold editions with a sports band attached. You'll be able to buy an additional sports band to use with them, but it won't come in the box.