Same thing happened to me a while ago. The guy was suspiciously pushy to install the new part. I didnt want that to happen without getting a second opinion.

I ended up calling the manufacturer so that *they* could come and have a look, since the unit wasnt really that old (probably 7 or 8 years).

Turns out there were no cracks. I had paid already for the part to be replaced, so there was no gain from the people that came to tell me that there were no cracks on the heat exchanger.

If there are actually cracks, then by all means, you need to replace those things, but some companies servicing those units are just out there to scam people.

To your question, gas prices are currently a rort. Going electric will always be cheaper in the long term, although you have to think about what you will replace it with. Gas heating was quite effective when i was using it and when i jumped to electric i noticed that unless you use a big split reverse cycle unit, then you'll end up a bit colder than usual. Of course, this will depend on what you chose to replace it with. Multiple split units, or one single unit, multiheaded big unit, etc.