taw1126,
I have purchased additional barrels and initially I settled on a not so personally desirable caliber in much the same way as you describe. I made a call to the gents at Mauser USA now based in San Antonio, TX (phone 210-377-2527) and in one case the barrel was on hand and took only 3-days to receive (a 9.3x62). The other a 7x64 was only a two week wait. Regarding cost it is a matter of what caliber you possess and which you desire. If your parent caliber and configuration matches the one in which you order it is only a matter of a barrel purchase, if however the two cartridges are not within the same family an order of magazine, bolt head and barrel would be required. Worst case scenario if all three would be ordered the current price is $1,300.00 USD. For more information on the family of cartridges simply type "Mauser Waffen" in your search engine and take a look at the Mauser web site- highly recommended as they provide a fair amount of information on the M03 and M-98 rifles with a few very good videos on the M-03 and its function. The gentlemen at Mauser USA were very good to discuss the finer points as well. Hope this is of some assistance- Best regards