Lets say you already own 5 accounts.
And plan to continue 5 boxing.

Just leveling toons on your existing accounts will be both the cheapest financially and the longest time investment.
$0 extra, but 25-30 hours of your time.

You could purchase 2 classic wow (not battlechests, BC, or Wrath) accounts.
Then RAF link from 2 existing accounts to the 2 new accounts.
Level (boost) 4 toons with one booster.
Then possibly character transfer from the 2 classic accounts, to two of your existing (and upgraded) accounts.
This costs you 2x Classic Wow + $50 (2 transfers at $25 each), but reduces 1-60th to 1/3rd normal time.
In the neighborhood of $100, but you save 16-20 hours of play time.

If you had purchased a slug of the $5 anniversary accounts.
It would be possible to level two keeper and two disposable characters.
And then repeat the process with two more of each.
And only cost $20, for 2/3rds the 1-60 time (save 8-10 hours).
If you were to buy 4x classic retail copies at the normal price, this might be close to $100.
But, if you were planning on boosting multiple sets of toons, this is cheaper then transfers.

It is always a balance between financial cost and time cost.
Purely for the RAF speed, combining with existing accounts, you only need classic wow.
But possibly character transfers too.