Pretty much all resorts can 'bring in' nitrox fills from another resort with a day or so of notice.
Pao made a very good point about the analyser though, make sure the resort has one available and check the sensor is in good condition - if it doesn't look too good ask if you can check the calibration using O2 (which most resorts should have for emergency use), a faulty sensor can be reasonable calibrated to read 20.9% for air - a bad sensor will show up if calibrated correctly for air and then used to analyse pure O2 - sensors.
The cost is generally similar across most resorts, from what I have seen the more expensive fills tend to come from better maintained equipment - this isn't always the rule it a good thing to bear in mind - most filling stations are intended to attract divers to the resort, not to make a killing on the fills themselves.
Hope that helps,
Karl