You would need a barrel and 2 Dolos. 2 uppers and 2 barrels. Such as 5.56 and 300 AAC.
If you have 5.56 you would need 2 Dolos, 170.00 x 2. Another 170.00 Barrel AAC. 510.00 total.
Vs 170.00 AAC barrel + 90 Upper. 260.00 total. 300 AAC Barrel to a new upper would be cheaper.
No change in BCG or mags between 300 AAC and 5.56. 50 Beowulf or something like that would also include a BCG. And mags labeled 50 Beowulf.
So to answer no it would not be a cheaper route. But the Dolos is about portability, not cheap change outs. Makes a nice Pilot Backpack survival gun. There is one on the market for that fits in a backpack and gives the option for carbine calibers, like 9, 45, etc.. can't remember the company. Has a quick attach barrel and fits into a backpack case. Comes with 2 or 3 barrels to change out. Depends on what the Pilot would carry as a sidearm. It was on Military Channel.