I handload in batches of 50rd or 100rd because that's the quantity the various components are packaged in; so I'd consider 100 as simply a beginning number, and the suggestion for 300 overall as a good total number. Barrel life for the 6.5CM probably tops out at around 2,000 to 2,500 rounds, which would divide by 300 to suggest 6 to 8 reloadings, which is attainable from Lapua brass.
Brass loss adds to the requirement.
Brass conservation can help with this. To accommodate this and maybe give some relief to the brass reloadings number, perhaps another 100 cases might help.
Consider also that the 6.5CM may become a favorite, so additional rifles/barrels in the chambering may become a reasonable option. In such a case, a total number of cases may not be such a serious consideration. Also consider rotating the cases to keep reloadings for each case relatively equivalent.
For me; reloading is not so much about money as about accuracy. Handloading allows ammunition to best deliver a given barrel's accuracy potential. If precision accuracy is the goal, cost becomes less significant a factor.
Lapua is really top shelf stuff. I've bought it when the chambering I was interested in was only available from them at the time. IMHO, it's a great choice for folks who have already eliminated all the other inconsistencies that prevent Championship performance. For the rest of us mere mortals, lesser brass is quite adequate. I normally buy
Starline.
Greg