A SS watch with the sport band is $652.91 after tax in CA. A SS watch with a Milanese or BCB is $761.91. So it's about $109.00 difference between the two to upgrade to either the BCB or the Milanese Loop.
If you buy the Sport band separately, it's $53.41. So the total for both if you start with the Milanese or BCB is 815.32.
If you buy the Sport first, then the Milanese of BCB, you end up at $815.32, but you get the case that band came in.
So yes, it should be better to start with the most basic model and buy nicer bands later assuming you buy everything at full Apple retail pricing and don't sell any bands for a profit.
In other words, if you buy a Milanese loop now, and sell the loop separately for a profit, then you go back and buy a sport band at a discount from a third party (ebay, CL), then you would end up in a better place than buying sport to start.