Many thanks guys for all this advice (Cusco, you're absolutely right - it was late and I didn't think - I am often frustrated that people don't search first).
It seems I had underestimated the flying involved. I had expected a checkout - but with a party of about 12, I expected to do 2 or 3 hours of cost-sharing flying, thus making up for the cost of the check out. I must admit I didn't know about regulation of airspace such as the minimum heights on the aerial corridors. It looks like the smart thing to do is go for the tour operators that slimslag refers to.
I've had a quick look on google and found one for $125 - I think I've found the local airport below - can someone confirm?
Once again, thanks to all for your comments.
Grand Canyon airport on Google maps?