Secondly, you don't have to spend the money required to obtain a visa, just have proof of those funds. So no point in including that in your daily visa cost calculations.
If you look for the most cost efficient option, well the right visa for the purpose of your stay, is probably the cheapest.
If you want to stay 60 days, a Single Entry Tourist Visa is cheaper then a free Visa Exempt & extension.
If you want to stay longer then 90 days, a Multiple Entry Tourist Visa is cheaper then 1 Single Entry Visa & extension.
If you want to stay a year, a NON-Immigrant Visa is more economical then several Tourist Visas and extensions.
If you arrive on the 1. December and fly out on the 1. January at 2am, you will go through immigration on the 31st December, so absolute no problem. At worst, you'll pay 500THB.
Only if you made the first 90 day reporting at an Immigration Office.
After that it's very easy to do on-line and only takes a couple of minutes. Then it takes between a few hours and several days to get the approval notification.