Spend The FULL PP Cost PER TARGET
It's clarified in the OG context
Text that uses the phrase "by spending a like amount of additional Power Points" has its roots in the first edition where it is used, for example, in the optional modifiers of the Bolt power:
Additional Bolts: The character may cast up to 3 bolts by spending a like amount of additional Power Points. This must be decided before the power is cast. The bolts may be spread among targets as the character chooses. Firing the additional bolts does not incur any attach penalties.
Additional Damage: The character may also increase the damage to 3d6 by doubling the Power Point cost per bolt. This may be combined with the additional bolts, so firing 3 bolts of 3d6 damage costs 6 Power Points -(Savage Worlds 1E Core Rulebook, p.84)
As you can see, the second avaliable modifier clarifies the matter, and seeing as "the bolts may be spread among targets as the character chooses," Additional Bolts in this case is very similar in meaning to Additional Targets, though the latter, and explicit application to the Fly power are lacking here.
The Deluxe Edition is unique in it's use of this wording for Fly but the context remains the same, as it is for Invisibility, whose function in that ruleset is clarified in the PEG official answers forum. (Thanks to ValhallaGH's answer for that link)
As ValhallaGH mentioned, the most recent edition does not include this wording, instead giving an explicit cost per additional recipient - for example, the Fly power in this edition costs 3 PP, but has the following "Additional Recipients" text:
ADDITIONAL RECIPIENTS (+2): The power may affect more than one target for 2 additional Power Points each. -(Savage Worlds Adventure Edition, p.158)
Which is clearly not the same text, and requires no clarification, though it's interesting that it's cheaper to do in that edition.
But What About the Maintenance Costs?
Interestingly (and perhaps conveniently) that appears to have been the real question in the official clarification linked above. In it, Clint confirmed that if a power that cost...
...5 PP to cast [and] 1 PP per round to maintain [...] should affect 3 targets, the initial costs would be 3x5=15 PP and the costs to maintain the power would be 3x1=3 PP per round
This confusion may have been part of the reason that the most recent edition includes the following text:
It costs 1 Power Point to maintain a power for its base Duration. If a power’s Duration is five rounds, for example, it can be maintained for another five rounds for 1 Power Point.
Maintenance is per target but ignores other Power Modifiers, so renewing boost Trait on three allies costs 3 Power Points and extends the effect of each another five rounds.
Unless a power says otherwise, the caster can terminate it as a free action. -(Savage Worlds Adventure Edition, p.150)
This means that extending Fly for 5 targets in that edition would cost only 5 PP, even though it's initial casting would cost 3(base) + 2 for each extra target: 11PP.