Let me explain it.

Normal air tank Consist of 20% Oxigen and 80% Nitrogen
(the person that told you its only oxigen does not have a slightest idea about diving and gas mixtures it would be deadly to use 100% oxigen.
100% oxigen is only used in technical diving in decompression stops and requiere years of trainning.

Nitrox The Oxigen can be up to 40% Oxigen to 60% Nitrogen
You can use 32% oxigen with 68 nitrogen depends on the type of diving you are making.

Trimix Is a mixture with Helium Nitrogen and Oxigen , they use Helium because at dephts the Nitrogen Narcosis start to happen and Helium helps to prevent Narcosis.

Heliox that mix only used when they are inside a chamber for really deep dives usually for petrolum companies for making repairs

The prices are really variable depends if you´re filling them in a dive shop or inside a boat, the price also changes depending on the gas blend you are using. Cylinder (tank) size.

To get a tank filled up with a mixture you need to show your credentials that verfify you´re trainned for using that mixture.