Cheapest O2 kit?

[prodigit]

For those who have a carburetor and need tuning, is there like a cheap kit, that you can buy, where you can just slide an O2 sensor in your exhaust pipe, and measure the A/F levels?

Most of the kits require you to screw in the O2 sensor in the exhaust pipe. But most scooters/motorcycles have no provision for that; and I don't want to damage my exhaust.

Those kits are all $100+ kits...

Any other solutions?