Depends on the vehicle. Only a few companies will insure motorbikes, and normally only big bikes. My buddy has a cbr650 I think he pays 12k baht a year for 1st class.
Personally, I always either leave my motorbikes in the Thai person's name I bought it from, or put it in a thai person's name. Have the seller sign the greenbook, sign a copy of their ID card with phone number, and fill out the sellers side of the transfer of ownership form. Leave the buyer side blank.
- you have to buy the mandatory minimum insurance when you pay your yearly tax. Other than that, most motorbikes does have 2nd or 1st class insurance.
Just walk away and find another place. If you're already having issues with the agent at this stage, imagine the issues you'll have over the next year.
Besides the potential of being taxed, shipping is going to be expensive too. For probably close to the same cost as the shipping, you could buy him a 2nd hand laptop here for $300