Can I convert my retirement visa to a NON-B visa in Thailand and what are the costs involved?

This is pretty confusing. If you were on a retirement visa and you left the country without a re-entry permit your retirement visa is gone. To get 60 days you must have travelled to a Thai consulate in another country and applied for a visa there. If you got 30 days when you entered Thailand you are on a visa exempt entry. You can change to a Non-B from either a tourist visa or a visa exempt but that is down to your future employer and how quickly they can get the documentation together, most employers want you to leave Thailand and apply for the Non-B elsewhere as there's less paperwork involved

Did you go to Laos and return? If that's the case you don't have any visa at all. You lost your retirement visa when you left and when you returned you got a 30 day visa exempt stamp, and you went and got a 30 day extension.

If you have a job lined up and your employer is prepared to sponsor your non-B visa, they need to provide you with a very large amount of paperwork for you to go to immigration and apply to convert your visa exempt to non-B.

If you're already on the 30 day extension, you probably don't have enough time as it takes most companies many weeks to gather all the paperwork.