There's very little safety inside bangkok if a huge quake were too happen. Your choice most likely won't matter. We can't control. Maybe a big open space can help you, but I don't see many people sleeping outside for that.
Question about the varying costs per country: When the time for the extension comes, where does that happen? If it's at an immigration office in Thailand, will they ask for 10,000 baht or the price you paid in your country?