Exactly. Of course there is a premium. If it were me running a programme like that, I'd be specifying a choice of the many netbooks/subnotebooks available now, for kids that age. Hell, I'd be jacking up external funding and just buying the shit out of a bunch of eee's or something since it's only a trial for one class.*
But that's not what was clearly specified in the choices the OP presented to us. She has a choice of Macs and that's it.
Tinkabelle: If you can find a lightly used (sub 6 months or so I'm thinking) Macbook on Trademe or something, you can still buy warranty upgrades from Apple for piece of mind, so may be able top save money over buying new.
*Actually I wouldn't because I don't believe the laptop-per-pupil model is suitable at this level. Barely even at highschool level for that matter, but that's another kettle of fish.