Don't ask Toyota la. Go find a generic spare parts shop. If I can find it cheaper will you come to my house and let me charge you to replace it?
Anyway, O2 sensors for smaller cars isn't as scary as the bigger cars. Speaking of which, my Nissan Sylphy O2 sensor costs RM200 at the spare parts shop near my house. I highly doubt the Vios will be more expensive.
1. If you want to check, you need a voltmeter and know how to read it.
2. This is a simple O2 sensor, most if not all mechs can do it.
3. Find a spanner that'll hold the new sensor snugly, WD40 the current sensor - let it soak in, spanner remove it (bit by bit, don't be too aggressive). Unplug it and replace with new sensor and installation is reverse of everything.
Very easy laaa... The biggest problem is finding a cheap sensor only.