$1/piece is the price of used Weatherby brass.
100 will probably last you a very long time in either the 243. However, if I had the chance I'd buy the Lapua brass and never regret it.
Of note, I can easily set up 20-30 rounds for just a load workup session. And, once I dial it in I wouldn't mind doing 100 of the same load in the same brass using the same components. I suspect that if you limit yourself to a smaller number you will find yourself running out of loaded rounds more often than you'd like.