There is precedent for a lower-price on a mass transfer of toons, and it's seen in something every player has to deal with: the base subscription price. Consider: you can buy one month of gametime for $15, three months for $14 per, or 6 months for $13 per.

For a guild transfer of 100 toons, it wouldn't be unreasonable for the per-toon rate to be reduced from $25 dollars to $22 dollars. Or if a single player could transfer all up-to-10 toons on an account for a $100-150 dollar fee.

They actually probably wouldn't lose money on this, since they'd probably convince people to transfer more toons total. A hypothetical example: Johnny is transferring his Warrior main and his Priest alt along with his guild which is switching servers due to server economics and to increase the pool of potential recruits. Fee: $50. He's got a at-or-near max level hunter, druid, and warlock, but only plays them sometimes and doesn't feel it's worth the effort to transfer them all. If Blizzard offered him a discount where he could transfer all his toons, including his even lower alts in the level 40-70 range, for let's suppose $125, Johnny would, and Blizzard would probably see a higher net profit.